update publishing scripts for central repository

[lrytz]

No description provided.

[lrytz]

I had to change the release workflow a bit. With the old oss.sonatype.org, there was the possibility to stage a release and test it by adding the staging resolver.

That is no longer possible after the central repository migration, as far as I can tell.

When building a release, we have a downstream job (https://github.com/scala/scala-dist) that builds the tarballs and such, and uploads them to the GitHub release. This can now only be done after the main maven release is on maven central.

Overall that's fine, one real downside is that the https://github.com/scala/scala-dist-smoketest job (which has a small sbt project to test the new release, and also tests the tarballs) now only runs after everything is published...