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_16/v210.go

@@ -34,7 +34,7 @@ func RemigrateU2FCredentials(x *xorm.Engine) error {
 		CreatedUnix     timeutil.TimeStamp `xorm:"INDEX created"`
 		UpdatedUnix     timeutil.TimeStamp `xorm:"INDEX updated"`
 	}
-	if err := x.Sync2(&webauthnCredential{}); err != nil {
+	if err := x.Sync(&webauthnCredential{}); err != nil {
 		return err
 	}