Unify java versions

[andrzejressel]

I was migrating zio-logging to latest version and I've noticed some inconsistencies regarding Java version. While most of them are minor the release one caught my eye.

This PR:

• Removes all references to Java 1.8
• Opens up JavaVersion trait and adds Java11/17 and 21 implementations
• ~Sets default release Java to 11 - I believe libraries are released for Java 11 anyway, because of "source/target" flags, but it looks strange when docs are saying that Java 17 is default for release.~ I've added more descriptive comment

~Shouldn't javacOptions be removed altogether to ensure that tests on latest Java are actually using the new features?~

[guizmaii]

@andrzejressel Thanks for your contribution 🙂