Delete the backport branch after merge

[Atemu]

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See NixOS/nixpkgs#128105

Before we invent any hacks around upstream, it'd be great if the backport action could just delete merged branches on its own.

[chancez]

https://github.com/peter-evans/create-pull-request?tab=readme-ov-file#delete-branch has a similar feature but it's got a few gotchas, FWIW.

[Atemu]

That behaviour is even better than automatically closing PRs IMHO. We don't need any unnecessary backport PRs hanging around either.