Remove Scala 2.13 builds in Jelly-JVM 2.0.0

[Ostrzyciel]

As noted here, Scala 2 support should be removed in the 1.2.0 release... but we are not having that release at all. Also, this does sound like a backwards-incompatible change (even if it was marked as only "kinda supported"), so I think it would be best to put this in a MAJOR release.

This should speed up CI dramatically and make maintaining the sbt setup much easier.

TODO:

• Clean up and simplify build.sbt
• Check if anything has to be removed from the CI workflow definitions.
• Remove all current mentions of Scala 2 in the docs and the page on Scala 2 builds.
• Add a note in the getting started guide for coders that Scala 2 was supported up until 1.0.x, with a link to 1.0.x docs.