Use forum.gitea.com instead of old URL (#31989) (#31992)

Backport #31989 by @lunny

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

modules/activitypub/user_settings_test.go

@@ -10,7 +10,7 @@ import (
 	"code.gitea.io/gitea/models/unittest"
 	user_model "code.gitea.io/gitea/models/user"
 
-	_ "code.gitea.io/gitea/models" // https://discourse.gitea.io/t/testfixtures-could-not-clean-table-access-no-such-table-access/4137/4
+	_ "code.gitea.io/gitea/models" // https://forum.gitea.com/t/testfixtures-could-not-clean-table-access-no-such-table-access/4137/4
 
 	"github.com/stretchr/testify/assert"
 )