!per #15436 make persistenceId abstract in NEW classes

(cherry picked from commit de3249f7f4b859c3caa232e579d9a3bae7406803)

Conflicts:
	akka-samples/akka-sample-persistence-scala/src/main/scala/sample/persistence/PersistentActorExample.scala
Merge pull request #15456 from akka/wip-15436-abstract-persistenceId-master-patriknw

!per #15436 make persistenceId abstract in NEW classes (for validation)