Use Java 21 LTS

John Duffell has reported improved speed from Java 21 LTS:

https://chat.google.com/room/AAAAag0I08g/Nvinl3aN_cU/

...and Mariot has confirmed that we should be encouraging people to move to Java 21.

In `gha-scala-library-release-workflow` we always use the same version of Java to _compile_ the project, regardless of the repo, because we've not yet encountered a need to configure the Java on a pre-repo basis. However, we do recommend that repos specify the `-release` flag in their `build.sbt` `scalacOptions`, which specifies the minimum release of Java that should be supported in the resulting artifacts - we still have some systems runinng on Java 8, so unfortunately they need `-release:8`

https://docs.scala-lang.org/overviews/jdk-compatibility/overview.html#jdk-21-compatibility-notes

.github/workflows/reusable-release.yml

@@ -377,7 +377,7 @@ jobs:
       - uses: actions/setup-java@v4
         with:
           distribution: corretto
-          java-version: 17
+          java-version: 21 # This will always be a long-term support (LTS) release
           cache: sbt # the issue described in https://github.com/actions/setup-java/pull/564 doesn't affect this step (no version.sbt)
       - name: Release
         env: