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_12/v121.go

@@ -12,5 +12,5 @@ func AddIsRestricted(x *xorm.Engine) error {
 		IsRestricted bool  `xorm:"NOT NULL DEFAULT false"`
 	}
 
-	return x.Sync2(new(User))
+	return x.Sync(new(User))
 }