Add Index to comment.dependent_issue_id (#27325) (#27340)

Backport #27325 by @JakobDev

This Column is missing index. It is used by
[issue_service.deleteIssue](https://github.com/go-gitea/gitea/blob/7ea2a910cebaf51cfd13c0941029c404e408ae54/services/issue/issue.go#L300).

Co-authored-by: JakobDev <jakobdev@gmx.de>

models/issues/comment.go

@@ -246,7 +246,7 @@ type Comment struct {
 	NewTitle         string
 	OldRef           string
 	NewRef           string
-	DependentIssueID int64
+	DependentIssueID int64  `xorm:"index"` // This is used by issue_service.deleteIssue
 	DependentIssue   *Issue `xorm:"-"`
 
 	CommitID        int64