Clean up same thread execution contexts #26690

* deprecate internal sameThread ec and use a new one for all internal use sites
* Use the respective Scala version standard library "same thread" ec 
* fallback to the old inline impl on 2.12 when reflection isn't possible

akka-stream/src/main/scala/akka/stream/impl/Sinks.scala

@@ -398,7 +398,7 @@ import scala.util.control.NonFatal
           .foreach {
             case NonFatal(e) => p.tryFailure(e)
             case _           => ()
-          }(akka.dispatch.ExecutionContexts.sameThreadExecutionContext)
+          }(akka.dispatch.ExecutionContexts.parasitic)
         p.future
       }
       override def cancel(): Unit = {
@@ -572,7 +572,7 @@ import scala.util.control.NonFatal
               failStage(e)
           }
         try {
-          sinkFactory(element).onComplete(cb.invoke)(ExecutionContexts.sameThreadExecutionContext)
+          sinkFactory(element).onComplete(cb.invoke)(ExecutionContexts.parasitic)
         } catch {
           case NonFatal(e) =>
             promise.failure(e)