Deprecate Hackage package

[infinisil]

Nixfmt is currently on Hackage, but I don't think that's necessary:

• While pkgs.nixfmt is currently being populated by Hackage, there's no reason this would be necessary to build a Haskell package in Nix. Indeed the new pkgs.nixfmt-rfc-style works without Hackage
• Nixfmt doesn't really have an API surface that would be useful enough on its own. I consider it an end-user application. If some useful and reusable API surface emerges from the codebase, we can still consider packaging that on Hackage.

So I'd propose deprecating the Hackage package and not updating it anymore. This way nixfmt on Hackage will always be the legacy Serokell version.

This will remove a bunch of maintenance burden from the formatter team.

Ping @Sereja313 @Lucus16 @yorickvP