=pro #15031 use sbt-unidoc plugin

Migrated to sbt-unidoc plugin. This uses the updated `sbt-unidoc` version which uses leak-fixed `genjavadoc`, so we are not leaking resources again.

Tested by doing a dry-run release and comparing generated documentation. Scala API documentation matches one-to-one.

However I could not make a diff of the Java API docs, because for some reason Java API docs generated on my machine before the change have missing classes in some hierarchies. So I compared it just by looking at http://doc.akka.io/japi/akka/snapshot/. It seems to be alright.

Also `validatePullRequest` task generates documentation as it did before the change.

This is part of the #15031 effort.