Add JDK target setting

[armanbilge]

This one is inspired by @ahjohannessen in armanbilge/fs2-grpc#1 (comment).

It would default to JDK 8, of course :) We could just hardcode this, but I think it would be nice if our settings plugin is friendly to applications that know they will be running on a newer JVM.

[armanbilge]

Found some settings related to this in cats-parse.
https://github.com/armanbilge/cats-parse/blob/7b524daf2abe63ca896656cfd36c13e9051476f6/build.sbt#L82

[ahjohannessen]

That seems like a great approach! :)

[armanbilge]

As always, Vasil is doing some magic in typelevel/cats-effect#2825 :)

What's interesting there is the use of the --release flag instead of (in addition to?) the usual -source/-target.

This seems good for the next release, since it pairs nicely with #138.

Edit: seems I may be muddling scalac flags with javac flags :)