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_16/v196.go

@@ -14,8 +14,8 @@ func AddColorColToProjectBoard(x *xorm.Engine) error {
 		Color string `xorm:"VARCHAR(7)"`
 	}
 
-	if err := x.Sync2(new(ProjectBoard)); err != nil {
-		return fmt.Errorf("Sync2: %w", err)
+	if err := x.Sync(new(ProjectBoard)); err != nil {
+		return fmt.Errorf("Sync: %w", err)
 	}
 	return nil
 }