Deprecate old syntax

Author: odersky

compiler/src/dotty/tools/dotc/parsing/Parsers.scala

@@ -3117,6 +3117,7 @@ object Parsers {
             case GIVEN =>
               val start = in.skipToken()
               if in.token == USCORE then
+                deprecationWarning(em"`given _` is deprecated in imports; replace with just `given`", start)
                 in.nextToken()
                 ImportSelector(givenSelectorId(start)) // Let the selector span all of `given _`; needed for -Ytest-pickler
               else if canStartTypeTokens.contains(in.token) then