contrib: Submit release PRs via user repo
contrib: Submit release PRs via user repo

Rather than depending on the ability to push release PRs via the main
cilium repo, rely on the release manager's repo for this instead.

Signed-off-by: Joe Stringer <16a9a54ddf4259952e3c118c763138e83693d7fd@cilium.io>

tgraf/cilium,tgraf/cilium,cilium/cilium,tgraf/cilium,tgraf/cilium,michi-covalent/cilium,cilium/cilium,cilium/cilium,michi-covalent/cilium,tklauser/cilium,michi-covalent/cilium,michi-covalent/cilium,tklauser/cilium,tklauser/cilium,michi-covalent/cilium,cilium/cilium,tklauser/cilium,cilium/cilium,tklauser/cilium,tgraf/cilium,tgraf/cilium