Refer to Scala 2.12 in docs; deprecate Scala 2.11 support in 2.4.1

Scala 2.11 support is removed in Spark 3.0, so we should at least call it deprecated in 2.4.x.

The 2.4.x docs current refer to Scala 2.11 artifacts. As 2.12 has been supported since 2.4.0 without any significant issues, we should refer to 2.12 artifacts in the docs by default as well.

You could say this implicitly declares it 'unexperimental', if it ever was deemed experimental, as we'd certainly support 2.12 and not change that support for the foreseeable future.