Update contributing with Java 15 requirement (elastic#69829)

The build has been updated to require JDK15 for compilation but the
contributing docs were out of date. This commit updates the docs to
have the correct JDK requirement.

Backport of elastic#69824

CONTRIBUTING.md

@@ -98,25 +98,25 @@ Contributing to the Elasticsearch codebase
 
 **Repository:** [https://github.com/elastic/elasticsearch](https://github.com/elastic/elasticsearch)
 
-JDK 14 is required to build Elasticsearch. You must have a JDK 14 installation
+JDK 15 is required to build Elasticsearch. You must have a JDK 15 installation
 with the environment variable `JAVA_HOME` referencing the path to Java home for
-your JDK 14 installation. By default, tests use the same runtime as `JAVA_HOME`.
+your JDK 15 installation. By default, tests use the same runtime as `JAVA_HOME`.
 However, since Elasticsearch supports JDK 8, the build supports compiling with
-JDK 14 and testing on a JDK 8 runtime; to do this, set `RUNTIME_JAVA_HOME`
+JDK 15 and testing on a JDK 8 runtime; to do this, set `RUNTIME_JAVA_HOME`
 pointing to the Java home of a JDK 8 installation. Note that this mechanism can
 be used to test against other JDKs as well, this is not only limited to JDK 8.
 
 > Note: It is also required to have `JAVA8_HOME`, `JAVA9_HOME`, `JAVA10_HOME`
-and `JAVA11_HOME`, and `JAVA12_HOME` available so that the tests can pass.
+and `JAVA11_HOME`, `JAVA12_HOME`, `JAVA13_HOME`, `JAVA14_HOME`, and `JAVA15_HOME`
+available so that the tests can pass.
 
 Elasticsearch uses the Gradle wrapper for its build. You can execute Gradle
 using the wrapper via the `gradlew` script on Unix systems or `gradlew.bat`
 script on Windows in the root of the repository. The examples below show the
 usage on Unix.
 
-We support development in IntelliJ versions IntelliJ 2019.2 and
-onwards. We would like to support Eclipse, but few of us use it and has fallen
-into [disrepair][eclipse].
+We support development in IntelliJ versions IntelliJ 2020.1 and
+onwards and Eclipse 2020-3 and onwards.
 
 [Docker](https://docs.docker.com/install/) is required for building some Elasticsearch artifacts and executing certain test suites. You can run Elasticsearch without building all the artifacts with:
 
@@ -138,9 +138,10 @@ and then run `curl` in another window like this:
 
 ### Importing the project into IntelliJ IDEA
 
-Elasticsearch builds using Java 14. When importing into IntelliJ you will need
+The minimum IntelliJ IDEA version required to import the Elasticsearch project is 2020.1
+Elasticsearch builds using Java 15. When importing into IntelliJ you will need
 to define an appropriate SDK. The convention is that **this SDK should be named
-"14"** so that the project import will detect it automatically. For more details
+"15"** so that the project import will detect it automatically. For more details
 on defining an SDK in IntelliJ please refer to [their documentation](https://www.jetbrains.com/help/idea/sdk.html#define-sdk).
 SDK definitions are global, so you can add the JDK from any project, or after
 project import. Importing with a missing JDK will still work, IntelliJ will
@@ -197,6 +198,78 @@ Note that only some sub-projects in the Elasticsearch project are currently
 fully-formatted. You can see a list of project that **are not**
 automatically formatted in [gradle/formatting.gradle](gradle/formatting.gradle).
 
+### Importing the project into Eclipse
+
+Elasticsearch builds using Gradle and Java 15. When importing into Eclipse you
+will either need to use an appropriate JDK to run Eclipse itself (e.g. by
+specifying the VM in [eclipse.ini](https://wiki.eclipse.org/Eclipse.ini) or by
+defining the JDK Gradle uses by setting **Preferences** > **Gradle** >
+**Advanced Options** > **Java home** to an appropriate version.
+
+IMPORTANT: If you have previously imported the project by running `./gradlew eclipse`
+           then you must build an entirely new workspace and `git clean -xdf` to
+           blow away *everything* that the gradle eclipse plugin made.
+
+ - Select **File > Import...**
+ - Select **Existing Gradle Project**
+ - Select **Next** then **Next** again
+ - Set the **Project root directory** to the root of your elasticsearch clone
+ - Click **Finish**
+
+This will spin for a long, long time but you'll see many errors about circular
+dependencies. Fix them:
+
+ - Select **Window > Preferences**
+ - Select **Java > Compiler > Building**
+ - Look under **Build Path Problems**
+ - Set **Circular dependencies** to **Warning**
+ - Apply that and let the build spin away for a while
+
+Next you'll want to import our auto-formatter:
+
+ - Select **Window > Preferences**
+ - Select **Java > Code Style > Formatter**
+ - Click **Import**
+ - Import the file at **buildSrc/formatterConfig.xml**
+ - Make sure it is the **Active profile**
+
+Finally, set up import order:
+
+ - Select **Window > Preferences**
+ - Select **Java > Code Style > Organize Imports**
+ - Click **Import...**
+ - Import the file at **buildSrc/elastic.importorder**
+ - Set the **Number of imports needed for `.*`** to ***9999***
+ - Set the **Number of static imports needed for `.*`** to ***9999*** as well
+ - Apply that
+
+IMPORTANT: There is an option in **Gradle** for **Automatic Project Synchronization**.
+           As convenient as it'd be for the projects to always be perfect this
+           tends to add many many seconds to every branch change. Instead, you
+           should manually right click on a project and
+           **Gradle > Refresh Gradle Project** if the configuration is out of
+           date.
+
+As we add more subprojects you might have to re-import the gradle project (the
+first step) again. There is no need to blow away the existing projects before
+doing that.
+
+### REST Endpoint Conventions
+
+Elasticsearch typically uses singular nouns rather than plurals in URLs.
+For example:
+
+    /_ingest/pipeline
+    /_ingest/pipeline/{id}
+
+but not:
+
+    /_ingest/pipelines
+    /_ingest/pipelines/{id}
+
+You may find counterexamples, but new endpoints should use the singular
+form.
+
 ### Java Language Formatting Guidelines
 
 Java files in the Elasticsearch codebase are formatted with the Eclipse JDT