for Scala 3, remove a type parameter from StreamConverters

I do not fully understand why this was here, but Scala 3 was
complaining: "Polymorphic refinement method valueStepper without
matching type in parent trait MapOps is no longer allowed"

simplifying the code as I have done in this commit compiles on
both Scala versions and the tests still pass, and this is an
obscure enough corner of the API that I'm not inclined to spend
time really digging deeply into it (e.g. attempting to construct
a test case that would now compile when it shouldn't, or vice
versa)

Author: SethTisue

Diff for: src/main/scala-2.13+/scala/compat/java8/StreamConverters.scala

@@ -86,7 +86,7 @@ trait StreamExtensions {
 
   implicit class MapHasParKeyValueStream[K, V, CC[X, Y] <: collection.MapOps[X, Y, collection.Map, _]](cc: CC[K, V]) {
     private type MapOpsWithEfficientKeyStepper = collection.MapOps[K, V, collection.Map, _] { def keyStepper[S <: Stepper[_]](implicit shape : StepperShape[K, S]) : S with EfficientSplit }
-    private type MapOpsWithEfficientValueStepper = collection.MapOps[K, V, collection.Map, _] { def valueStepper[V1 >: V, S <: Stepper[_]](implicit shape : StepperShape[V1, S]) : S with EfficientSplit }
+    private type MapOpsWithEfficientValueStepper = collection.MapOps[K, V, collection.Map, _] { def valueStepper[S <: Stepper[_]](implicit shape : StepperShape[V, S]) : S with EfficientSplit }
     private type MapOpsWithEfficientStepper = collection.MapOps[K, V, collection.Map, _] { def stepper[S <: Stepper[_]](implicit shape : StepperShape[(K, V), S]) : S with EfficientSplit }
 
     /** Create a parallel [[java.util.stream.Stream Java Stream]] for the keys of this map. If