Add Index to comment.dependent_issue_id (#27325)

This Column is missing index. It is used by
[issue_service.deleteIssue](7ea2a910ce/services/issue/issue.go (L300)).

Co-authored-by: Giteabot <teabot@gitea.io>

models/migrations/v1_21/v278.go

@@ -0,0 +1,16 @@
+// Copyright 2023 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package v1_21 //nolint
+
+import (
+	"xorm.io/xorm"
+)
+
+func AddIndexToCommentDependentIssueID(x *xorm.Engine) error {
+	type Comment struct {
+		DependentIssueID int64 `xorm:"index"`
+	}
+
+	return x.Sync(new(Comment))
+}