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/v215.go

@@ -20,5 +20,5 @@ func AddReviewViewedFiles(x *xorm.Engine) error {
 		UpdatedUnix  timeutil.TimeStamp          `xorm:"updated"`
 	}
 
-	return x.Sync2(new(ReviewState))
+	return x.Sync(new(ReviewState))
 }