reflect/protodesc: fix group validation: not supported in proto3

Before this change, calling dynamicpb.NewMessage() on a
protoreflect.MessageDescriptor that uses Editions would fail with:

message field "[…].relatedqueries" is an invalid group: proto: invalid under
proto2 semantics

(While Go Protobuf does not yet support Editions, C++ programs
can save descriptors to a file that is later loaded by a Go program.)

Given that groups are only supported in proto2, but not in proto3
(see https://protobuf.dev/editions/features/#message_encoding),
I think the conditional meant to say "proto3 semantics".

Change-Id: I00368e4433ca60bb883b726e34ee6b8862cfd1e7
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/554955
Reviewed-by: Cassondra Foesch <[email protected]>
Reviewed-by: Christian Höppner <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

reflect/protodesc/desc_validate.go

@@ -314,8 +314,8 @@ func checkValidGroup(fd protoreflect.FieldDescriptor) error {
 	switch {
 	case fd.Kind() != protoreflect.GroupKind:
 		return nil
-	case fd.Syntax() != protoreflect.Proto2:
-		return errors.New("invalid under proto2 semantics")
+	case fd.Syntax() == protoreflect.Proto3:
+		return errors.New("invalid under proto3 semantics")
 	case md == nil || md.IsPlaceholder():
 		return errors.New("message must be resolvable")
 	case fd.FullName().Parent() != md.FullName().Parent():