Add a custom `mill` task for publishing sonatype artifacts

[Gedochao]

My heart bleeds :<

Could we add a custom task that does it instead?

Originally posted by @tgodzik in #2819 (comment)

We indeed should have a custom mill task for publishing sonatype artifacts, as the syntax used on our CI currently is quite ugly.
Doing so will simplify bumping Scala 2.12 & 2.13 versions (not to mention making it more futureproof).