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_9/v84.go

@@ -13,5 +13,5 @@ func AddGPGKeyImport(x *xorm.Engine) error {
 		Content string `xorm:"TEXT NOT NULL"`
 	}
 
-	return x.Sync2(new(GPGKeyImport))
+	return x.Sync(new(GPGKeyImport))
 }