Add Index to pull_auto_merge.doer_id (#27811)

Reported at https://codeberg.org/forgejo/forgejo/issues/1677

This column is used by
[deleteUser](f089be91da/services/user/delete.go (L90)).

Note: This PR contains a Migration, so we can't backport it to 1.21.

models/migrations/v1_22/v282.go

@@ -0,0 +1,16 @@
+// Copyright 2023 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package v1_22 //nolint
+
+import (
+	"xorm.io/xorm"
+)
+
+func AddIndexToPullAutoMergeDoerID(x *xorm.Engine) error {
+	type PullAutoMerge struct {
+		DoerID int64 `xorm:"INDEX NOT NULL"`
+	}
+
+	return x.Sync(&PullAutoMerge{})
+}