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 https://github.com/lampepfl/dotty/issues/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