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/db/engine.go

@@ -55,7 +55,7 @@ type Engine interface {
 	Limit(limit int, start ...int) *xorm.Session
 	NoAutoTime() *xorm.Session
 	SumInt(bean any, columnName string) (res int64, err error)
-	Sync2(...any) error
+	Sync(...any) error
 	Select(string) *xorm.Session
 	NotIn(string, ...any) *xorm.Session
 	OrderBy(any, ...any) *xorm.Session
@@ -172,7 +172,7 @@ func UnsetDefaultEngine() {
 }
 
 // InitEngineWithMigration initializes a new xorm.Engine and sets it as the db.DefaultContext
-// This function must never call .Sync2() if the provided migration function fails.
+// This function must never call .Sync() if the provided migration function fails.
 // When called from the "doctor" command, the migration function is a version check
 // that prevents the doctor from fixing anything in the database if the migration level
 // is different from the expected value.

models/db/engine_test.go

@@ -26,7 +26,7 @@ func TestDumpDatabase(t *testing.T) {
 		ID      int64 `xorm:"pk autoincr"`
 		Version int64
 	}
-	assert.NoError(t, db.GetEngine(db.DefaultContext).Sync2(new(Version)))
+	assert.NoError(t, db.GetEngine(db.DefaultContext).Sync(new(Version)))
 
 	for _, dbType := range setting.SupportedDatabaseTypes {
 		assert.NoError(t, db.DumpDatabase(filepath.Join(dir, dbType+".sql"), dbType))