docs: `check-supported` is outdated

[Malix-Labs]

flake-checker-action/README.md

Line 42 in 9ee1c54

| `check-supported` | Whether to check that the root Nixpkgs input has a supported Git ref. Currently supported refs: `nixos-22.11`, `nixos-22.11-small`, `nixos-23.05`, `nixos-23.05-small`, `nixos-unstable`, `nixos-unstable-small`, `nixpkgs-22.11-darwin`, `nixpkgs-23.05-darwin`, `nixpkgs-unstable`. | `true` |

is outdated

https://github.com/DeterminateSystems/flake-checker/blob/main/ref-statuses.json

[lucperkins]

Ah, good call. I haven't yet applied the templating logic to that list, but now would be a great time to do that.