Rename Sync2 -> Sync (#26479)

The xorm `Sync2` has already been deprecated in favor of `Sync`,
so let's do the same inside the Gitea codebase.

Command used to replace everything:
```sh
for i in $(ag Sync2 --files-with-matches); do vim $i -c ':%sno/Sync2/Sync/g' -c ':wq'; done
```

models/migrations/v1_11/v104.go

@@ -15,7 +15,7 @@ func RemoveLabelUneededCols(x *xorm.Engine) error {
 		QueryString string
 		IsSelected  bool
 	}
-	if err := x.Sync2(new(Label)); err != nil {
+	if err := x.Sync(new(Label)); err != nil {
 		return err
 	}