PR testing should be changed so it doesn't use out-of-support JDK20

[sxa]

What are you trying to do? Test PRs in this reposoitory

Expected behaviour: Tests things that we build

Observed behaviour: PR test tests JDK20, not 21 or the upcoming 21

Any other comments: PR testers job; https://ci.adoptium.net/job/build-scripts-pr-tester/job/openjdk-build-pr-tester/