fix: use READ COMMITTED transactions on MySQL and MariaDB (#39506)

MariaDB 11.6.2+ defaults `innodb_snapshot_isolation` to `ON`, which
fails REPEATABLE READ transactions with error 1020 when a row they write
changed after their first read. Gitea's background work like push
processing writes the same rows, so merges, issue closes and workflow
runs fail sporadically.

- Use READ COMMITTED on MySQL and MariaDB, like PostgreSQL and MSSQL
- Update xorm to v1.4.3

Replaces: https://github.com/go-gitea/gitea/pull/39494
Fixes: https://github.com/go-gitea/gitea/issues/39492

---------

Signed-off-by: silverwind <me@silverwind.io>

models/db/engine_init.go

@@ -5,6 +5,7 @@ package db
 
 import (
 	"context"
+	"database/sql"
 	"fmt"
 
 	"gitea.dev/modules/log"
@@ -59,6 +60,11 @@ func InitEngine(ctx context.Context) error {
 	xe.SetMaxIdleConns(setting.Database.MaxIdleConns)
 	xe.SetConnMaxLifetime(setting.Database.ConnMaxLifetime)
 
+	if setting.Database.Type.IsMySQL() {
+		// like PostgreSQL and MSSQL, avoids MariaDB snapshot isolation errors
+		xe.SetDefaultTxOptions(&sql.TxOptions{Isolation: sql.LevelReadCommitted})
+	}
+
 	if setting.Database.SlowQueryThreshold > 0 {
 		xe.AddHook(&EngineHook{
 			Threshold: setting.Database.SlowQueryThreshold,