EOL cleanup

The old release reaches its end of life one month after the new release gets released. At that point a few cleanup tasks need to be done.

1. Set the EOL channel status from deprecated to unmaintained in infra. Examples:

   • channels: Deprecate NixOS 22.05
   • channels: Mark 21.11 as unmaintained
   • channels:21.05 is unmaintained

2. Once this is merged, update the nixos-search repository to reflect that status on search.nixos.org.

   nix --extra-experimental-features "nix-command flakes" flake lock --update-input nixos-infra
   

3. Close all pull requests that target one of the the old release- or staging-branches

4. Prune old backport labels

5. Remove the old release from the periodic merge workflows

6. Increase the oldestSupportedRelease in lib/trivial.nix to match the oldest supported release.

   Examples: 22.05