chore(tool-versions): use current Java, Maven versions (#3358)

* chore(tool-versions): use current Java and Maven

* docs(readme): mention asdf, Java 17 in SDK

(cherry picked from commit 83d7157)

.tool-versions

@@ -1,2 +1,2 @@
-java temurin-17.0.12+7
-maven 3.8.4
+java temurin-21.0.1+12.0.LTS
+maven 3.9.9

README.md

@@ -14,6 +14,7 @@ For more information on Connectors, refer to the
 ## Contents
 
 * [License](#license)
+* [Setup](#setup)
 * [Create a Connector](#create-a-connector)
     * [Outbound Connector](#outbound-connector)
     * [Inbound Connector](#inbound-connector)
@@ -41,6 +42,15 @@ This is a multi-module project with different licenses applied to different modu
 
 When in doubt, refer to the `LICENSE` file in the respective module.
 
+## Setup
+
+Download these utilities:
+
+* [`make`](https://www.gnu.org/software/make/) for building
+* [`asdf`](https://asdf-vm.com/) for managing Java and Maven versions
+
+The [Connector SDK](connector-sdk) uses Java 17, unlike the rest of this repository.
+
 ## Create a Connector
 
 Include the [connector-core](connector-sdk/core), e.g. via Maven: