fix: Do not make deferred given implementations final

It should be possible to extend a class that inherits a deferred given,
so a generated given implementation should not be final.

compiler/src/dotty/tools/dotc/typer/Typer.scala

@@ -3038,7 +3038,7 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
           body
 
     /** Implement givens that were declared with a `deferred` rhs.
-     *  The a given value matching the declared type is searched in a
+     *  The given value matching the declared type is searched in a
      *  context directly enclosing the current class, in which all given
      *  parameters of the current class are also defined.
      */
@@ -3067,7 +3067,7 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
             )(using searchCtx)
 
           val impl = dcl.copy(cls,
-            flags = dcl.flags &~ (HasDefault | Deferred) | Final | Override,
+            flags = dcl.flags &~ (HasDefault | Deferred) | Override,
             info = target,
             coord = rhs.span).entered.asTerm
 

tests/pos/i21189.scala

@@ -0,0 +1,10 @@
+//> using options -source:future -language:experimental.modularity
+
+class MySortedSet[T : Ord] extends SortedSet[T]
+
+trait Ord[T]
+
+trait Sorted[T]:
+  given Ord[T] as ord = compiletime.deferred
+
+class SortedSet[T : Ord] extends Sorted[T]