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/issues/content_history_test.go

@@ -46,7 +46,7 @@ func TestContentHistory(t *testing.T) {
 		Name     string
 		FullName string
 	}
-	_ = db.GetEngine(dbCtx).Sync2(&User{})
+	_ = db.GetEngine(dbCtx).Sync(&User{})
 
 	list1, _ := issues_model.FetchIssueContentHistoryList(dbCtx, 10, 0)
 	assert.Len(t, list1, 3)