GitHub Actions config: JDK 20 is now generally available

[SethTisue]

I'm testing this in PR validation first (with a commit adding pull-request: that I'll drop before merge) so we can be sure that the JDK 20 builds are actually available through GitHub Actions now. They are definitely available from the Temurin site.

If this succeeds, I should merge 2.12.x onto 2.13.x.

[SethTisue]

the Java installation section looks good in both the Ubuntu and Windows logs. Ubuntu log has:

  Java configuration:
    Distribution: temurin
    Version: 20.0.0+36
    Path: /opt/hostedtoolcache/Java_Temurin-Hotspot_jdk/20.0.0-36/x64

[SethTisue]

can't figure out how to let GitHub allow me to force push the branch to remove the extra commit — even by making a temporary settings change that I'd then change back. okay, I'll make a new PR 🤷