Add Index to pull_auto_merge.doer_id (#27811)

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

This column is used by
[deleteUser](https://github.com/go-gitea/gitea/blob/f089be91da44ee3e85fbe326a204c7c8a5df3d31/services/user/delete.go#L90).

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

models/migrations/migrations.go

@@ -548,6 +548,8 @@ var migrations = []Migration{
 	NewMigration("Rename user themes", v1_22.RenameUserThemes),
 	// v281 -> v282
 	NewMigration("Add auth_token table", v1_22.CreateAuthTokenTable),
+	// v282 -> v283
+	NewMigration("Add Index to pull_auto_merge.doer_id", v1_22.AddIndexToPullAutoMergeDoerID),
 }
 
 // GetCurrentDBVersion returns the current db version