Stop testing JDK 8

[saeltz]

The next version of Scala 3 won’t support JDK 8 anymore. We should consider dropping our explicit tests.

[mccartney]

I think we are good with this. Even with older versions (Scala 2.x.x) I don't recall any issue with JDK 8 incompatibility.
At the same time - we need to make sure the release continues to use JDK 8 to build our artifacts.

[saeltz]

Would we still be able to release on JDK 8 if the compiler doesn’t support that version anymore?

[Johnnei]

At least for standard java, using the --release flag still support java 8 bytecode/classpath compilation on Java 21. As most new baselines seem to be set at 17, I think we're fine for the time being. Wouldn't expect the Scala compiler to be much aggressive here except for Scala 3.8+.

I thought I read somewhere that post jdk 21 it would be dropped, but can't find that reference back. Maybe it was just an off handed comment on the mailing lists.