Replacing JDK20 with JDK21 in pipelines

[adamfarley]

There will be no more JDK20 releases, and JDK21 is next to be released, so this PR does a swap.

Also includes some improvements to generateBuildMatrix.sh that makes the code and the output more readable.

[github-actions]

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[adamfarley]

I performed a more in-depth replacement than usual as JDK21 is an LTS, and therefore more likely to be around longer.

[andrew-m-leonard]

looks good, thanks Adam

[andrew-m-leonard]

run tests quick

[eclipse-temurin-bot]

 PR TESTER RESULT 

✅ All pipelines passed! ✅