Evaluate, retrospect on and update the PR tester

[sxa]

The PR tester job (docs are here) is somewhat neglected and hasn't had some updates for a while so now feels like a good time to look at a bit of a retrospective/overhaul. As a discussion starter some specific issues I note are:

• It includes JDK20 which is out of support
• It does not include the head stream (and possibly one release from a branch in the jdk repository to trap issues with the new branching process upstream)
• It should possible include JDK8/arm32 which is "unusual" in coming from a separate codebase.
• Maybe include AIX as well for some versions?
• Could we encourage its use for testing things in temurin-build too, particularly if it is modified to include some of the additional platforms not covered by the existing github actions checks?
• It is generating unnecessary warnings in the jenkins logs. I believe the jobs should be generated automatically at the start of each run so that theoretically shouldn't be a problem.

We should use this as an opportunity to see if there are specific scenarios/platforms that it would be useful to include in the PR tester to find any problems that we see, so perhaps a brainstorming session is in order. Perhaps JDK8 should include arm32 and Solaris, and potentially other versions can include AIX as additional checks.