Disable federation by default (#20045) (#20046)

* Disable federation by default (#20045)

- Backport #20045
  - A Gitea instance should choose whetever they want to federate(as once it has more features also brings extra costs/moderation/unexpected behavior) with other AP/ForgeFed software.

* Fix tests

modules/setting/federation.go

@@ -12,7 +12,7 @@ var (
 		Enabled             bool
 		ShareUserStatistics bool
 	}{
-		Enabled:             true,
+		Enabled:             false,
 		ShareUserStatistics: true,
 	}
 )