=act #21774 fix ByteString.toString to use fixed stringPrefix for all subclasses (#21775)

Otherwise, with 2.12,

ByteString().toString == "ByteString.ByteString1C()"

which would expose implementation details in the string representation.
This can lead to failing tests due to test expecting a particular string
representation of a ByteString which might be bad practice, yes, but is also
convenient.

The change is due to a fix in Scala for SI-9019 for which the string representation
of TraversableLike was changed which ByteString inherits.
See https://github.com/scala/scala/pull/5258/files

project/MiMa.scala

@@ -1018,7 +1018,10 @@ object MiMa extends AutoPlugin {
 
       ),
       "2.4.12" -> Seq(
-        ProblemFilters.exclude[ReversedMissingMethodProblem]("akka.stream.Materializer.materialize")
+        ProblemFilters.exclude[ReversedMissingMethodProblem]("akka.stream.Materializer.materialize"),
+        
+        // #21775 - overrode ByteString.stringPrefix and made it final
+        ProblemFilters.exclude[FinalMethodProblem]("akka.util.ByteString.stringPrefix")
       )
     )
   }