Don't check for scala versions anymore for scala native

[Philippus]

I think this will help solve the empty jars problem in #444. I had to update scala 3.0.2 to 3.1.1 as well to make the build succeed.

[SethTisue]

I think it's fine to use Scala 3.1. It means users of our next library version won't be able to use it on Scala 3.0, but I think that's fine, let them move to 3.1. (Scala 3.1.2-RC1 adds -Yscala-release which aims to add this kind of forward-compatibility, but I think it's too new/experimental for us to rely on.)

I think the check for JDK 1.6 can be dropped; we no longer build or test on JDK 6 or 7.

I'm curious if the test := {} is still needed.

Did you look to see if publishLocal publishes non-empty JARs?

[Philippus]

publishLocal indeed published empty jars, after this change not anymore.
I've dropped jdk 6 and enabled the tests for scala native.