Add build_flavor back to info api rest response (elastic#88336)

The build_flavor was previously removed since it is no longer relevant;
only the default distribution now exists. However, the removal of build
flavor included removing it from the version information on the info
response for the root path. This API is supposed to be stable, so
removing that key was a compatibility break. This commit adds the
build_flavor back to that API, hardcoded to `default`. Additionally, a
test is added to ensure the key exists going forward, until it can be
properly deprecated.

closes elastic#88318

build.gradle

@@ -138,9 +138,9 @@ tasks.register("verifyVersions") {
  * after the backport of the backcompat code is complete.
  */
 
-boolean bwc_tests_enabled = true
+boolean bwc_tests_enabled = false
 // place a PR link here when committing bwc changes:
-String bwc_tests_disabled_issue = ""
+String bwc_tests_disabled_issue = "https://github.com/elastic/elasticsearch/pull/88336"
 if (bwc_tests_enabled == false) {
   if (bwc_tests_disabled_issue.isEmpty()) {
     throw new GradleException("bwc_tests_disabled_issue must be set when bwc_tests_enabled == false")

docs/changelog/88336.yaml

@@ -0,0 +1,6 @@
+pr: 88336
+summary: Add `build_flavor` back to info api rest response
+area: Infra/Core
+type: bug
+issues:
+ - 88318

docs/plugins/discovery-azure-classic.asciidoc

@@ -352,6 +352,7 @@ This command should give you a JSON result:
     "number" : "{version_qualified}",
     "build_type" : "{build_type}",
     "build_hash" : "f27399d",
+    "build_flavor": "default",
     "build_date" : "2016-03-30T09:51:41.449Z",
     "build_snapshot" : false,
     "lucene_version" : "{lucene_version}",

docs/reference/setup/install/check-running.asciidoc

@@ -36,6 +36,7 @@ GET /
     "number" : "{version_qualified}",
     "build_type" : "{build_type}",
     "build_hash" : "f27399d",
+    "build_flavor" : "default",
     "build_date" : "2016-03-30T09:51:41.449Z",
     "build_snapshot" : false,
     "lucene_version" : "{lucene_version}",

rest-api-spec/src/yamlRestTest/resources/rest-api-spec/test/info/10_info.yml

@@ -7,3 +7,11 @@
     - is_true:    tagline
     - is_true:    version
     - is_true:    version.number
+
+---
+"Info build flavor":
+    - skip:
+        version: "8.3.0 - 8.3.2"
+        reason: "build flavor in info was missing in 8.3.0 to 8.3.2"
+    - do:         {info: {}}
+    - match:      { version.build_flavor: default }

server/src/main/java/org/elasticsearch/action/main/MainResponse.java

@@ -87,6 +87,7 @@ public XContentBuilder toXContent(XContentBuilder builder, Params params) throws
         builder.field("cluster_uuid", clusterUuid);
         builder.startObject("version")
             .field("number", build.qualifiedVersion())
+            .field("build_flavor", "default")
             .field("build_type", build.type().displayName())
             .field("build_hash", build.hash())
             .field("build_date", build.date())

server/src/test/java/org/elasticsearch/action/main/MainResponseTests.java

@@ -64,6 +64,7 @@ public void testToXContent() throws IOException {
                         "cluster_uuid": "%s",
                         "version": {
                             "number": "%s",
+                            "build_flavor": "default",
                             "build_type": "%s",
                             "build_hash": "%s",
                             "build_date": "%s",