Re-enable versionPolicyCheck

[ashawley]

Discussing https://github.com/scala/bug/issues/12632 I was reminded that the Mima plugin and the sbt-version-policy plugin was relaxed and put back and relaxed again a couple of times:

• #551
• #598
• #604

It should probably be turned back on or at the very least conditionally so.

Currently it is:

    // should be reverted to Compatibility.BinaryAndSourceCompatible after 2.2.0 is released
    versionPolicyIntention := Compatibility.None,

[ashawley]

I'm less familiar with the build, but I believe:

1. it should be able to be turned back on for Java,
2. but can only be BinaryCompatible for Scala.js
3. and can't be checked for Native.

[SethTisue]

agree