=rem #13783 Make ProtobufSerializer independent of protobuf version

* well, as long as they provide the parseFrom and toByteArray
* it is using reflection to find the `parseFrom` and `toByteArray` methods to avoid
  dependency to `com.google.protobuf`.
* also special case com.google.protobuf when loading serialization binding
* migration guide
* mima filters for the serializers (all types changed)
* add real test for ProtobufSerializer

akka-actor/src/main/scala/akka/serialization/Serialization.scala

@@ -205,7 +205,15 @@ class Serialization(val system: ExtendedActorSystem) extends Extension {
    *  It is primarily ordered by the most specific classes first, and secondly in the configured order.
    */
   private[akka] val bindings: immutable.Seq[ClassSerializer] =
-    sort(for ((k: String, v: String) ← settings.SerializationBindings if v != "none") yield (system.dynamicAccess.getClassFor[Any](k).get, serializers(v))).to[immutable.Seq]
+    sort(for ((k: String, v: String) ← settings.SerializationBindings if v != "none" && checkGoogleProtobuf(k))
+      yield (system.dynamicAccess.getClassFor[Any](k).get, serializers(v))).to[immutable.Seq]
+
+  // com.google.protobuf serialization binding is only used if the class can be loaded,
+  // i.e. com.google.protobuf dependency has been added in the application project.
+  // The reason for this special case is for backwards compatibility so that we still can
+  // include "com.google.protobuf.GeneratedMessage" = proto in configured serialization-bindings.
+  private def checkGoogleProtobuf(className: String): Boolean =
+    (!className.startsWith("com.google.protobuf") || system.dynamicAccess.getClassFor[Any](className).isSuccess)
 
   /**
    * Sort so that subtypes always precede their supertypes, but without