github/update-flake-lock: only update master and latest release

Older releases are not supported upstream,
so the nixpkgs input shouldn't update,
most of the time

.github/workflows/update-flake-lock.yml

@@ -20,8 +20,6 @@ jobs:
         branch:
           - master
           - nixos-24.05
-          - nixos-23.11
-          - nixos-23.05
     permissions:
       contents: write # to create branch
       pull-requests: write # to create PR to backport