Update to newer nixpkgs and flakes

[MMesch]

No description provided.

[robwhitaker]

Thanks for the PR @MMesch! Sorry for taking so long to notice it; I didn't see any notification for it in my email. I actually just came back to this repo thinking I should update it since I started using Taskwarrior again, and there was a PR waiting doing just that!

Just a couple notes before I merge it:

1. Would you mind updating the flake.nix description to something non-default? The synopsis in the cabal file or the repo description is fine.
2. Is there a reason to bring in hls-splice-plugin and hls-eval-plugin in the devShell? Are they not brought along inherently with haskell-language-server? Even if not, I don't think there is any code in this project that benefits from the extra plugins so I'd rather not include them.

Other than that, it all looks good!