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_15/v182.go

@@ -20,7 +20,7 @@ func AddIssueResourceIndexTable(x *xorm.Engine) error {
 		return err
 	}
 
-	if err := sess.Table("issue_index").Sync2(new(ResourceIndex)); err != nil {
+	if err := sess.Table("issue_index").Sync(new(ResourceIndex)); err != nil {
 		return err
 	}