feat(repo): allow deleting protected branches (#39243)

This adds a Branch Deletion setting with three modes: disabled by
default, anyone with push access, or a restricted user/team/deploy-key
allowlist. The policy applies to Git pushes, API/UI deletion, and
post-merge cleanup; the default branch and the pull request target
branch remain undeletable.

Closes https://github.com/go-gitea/gitea/issues/33305

---------

Co-authored-by: Enoque Duarte <12515551+enoquefcd@users.noreply.github.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

services/forms/repo_form.go

@@ -159,6 +159,9 @@ type ProtectBranchForm struct {
 	EnableForcePush               string
 	ForcePushAllowlistUsers       string
 	ForcePushAllowlistTeams       string
+	EnableDeletion                string
+	DeletionAllowlistUsers        string
+	DeletionAllowlistTeams        string
 	EnableMergeWhitelist          bool
 	MergeWhitelistUsers           string
 	MergeWhitelistTeams           string