Remove push protection from backport branches

[roberth]

Describe the bug

Pushing to backport branches is a good way to fix backports that don't pass CI.

Steps To Reproduce

1. Try to push to backport-7616-to-2.13-maintenance

2. [...]
   remote: Resolving deltas: 100% (18/18), completed with 6 local objects.
   remote: error: GH006: Protected branch update failed for refs/heads/backport-7616-to-2.13-maintenance.
   remote: error: Changes must be made through a pull request. Cannot force-push to this branch
   To https://github.com/NixOS/nix.git
   ! [remote rejected]     backport-7616-to-2.13-maintenance -> backport-7616-to-2.13-maintenance (protected branch hook declined)
   error: failed to push some refs to 'https://github.com/NixOS/nix.git'

Expected behavior

Team members can push to backport branches.

nix-env --version output

Additional context

Priorities

Add :+1: to issues you find important.

[nixos-discourse]

This issue has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/2023-03-03-nix-team-meeting-minutes-37/25998/1