build: Ensure to produce JDK 8 compatible bytecode

[WojciechMazur]

Compiler version

3.4.nightly

With the recent changes to use JDK 17/21 in the CI we need to ensure that published artifacts and referenced API would be JDK 8 compatible. Otherwise we risk publishing with unexpected JDK bytecode version, as it was already found in some nightly releases #19721

Expectation

Modify the build.sbt to ensure that -target/-java-output-version / -release flags are set correctly. Possibly add dependency to publish tasks to fail compilation if required javac/scalac options are missing or are incorrect

[hamzaremmal]

We just discussed this during a meeting today. I will just add a big warning in the CI workflow mentioning that the java version should not change for the releases and community build

[reid-spencer]

I would prefer that the compiler generate code for the most recent JDK it is compatible with, but accept a command line option, say -java-output-version as suggested, to force it to (try to) generate code for older JDKs.

@WojciechMazur - While broad JDK version support is laudable, isn't it time to move on from 2014 (now a full decade out of date) and benefit from all the JDK improvements since 8? I am advocating that Scala consider what its minimum JDK version should be. I'd vote for 11. The Java/Scala/xxx community needs to forget about JDK 8. IMHO.