=per #15230 deprecate Channel and PersistentChannel

Conflicts:
	akka-persistence/src/main/scala/akka/persistence/Channel.scala
	akka-persistence/src/main/scala/akka/persistence/PersistentChannel.scala
	akka-persistence/src/test/scala/akka/persistence/serialization/SerializerSpec.scala
Merge pull request #15435 from ktoso/port-deprecations-channel-ktoso

!per #15230 deprecate Channel and PersistentChannel (for Validation)