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_17/v218.go

@@ -48,5 +48,5 @@ func (*improveActionTableIndicesAction) TableIndices() []*schemas.Index {
 }
 
 func ImproveActionTableIndices(x *xorm.Engine) error {
-	return x.Sync2(&improveActionTableIndicesAction{})
+	return x.Sync(&improveActionTableIndicesAction{})
 }