Bump hakyll upper bounds and niv updates

[philderbeast]

Fixes #108. Bumps the upper bound on hakyll.

The change also bumps ghc and cabal in the nix-shell too. With the cabal change we can start using a project and simplify the cabal-specific instructions in the readme.

  $ nix-shell

  [nix-shell:~/.../www.haskell.org]$ ghc --version
- The Glorious Glasgow Haskell Compilation System, version 8.8.4
+ The Glorious Glasgow Haskell Compilation System, version 9.6.5

  [nix-shell:~/.../www.haskell.org]$ cabal --version
- cabal-install version 3.2.0.0
- compiled using version 3.2.0.0 of the Cabal library 
+ cabal-install version 3.10.3.0
+ compiled using version 3.10.3.0 of the Cabal library

[rebeccaskinner]

Reviewed and tested locally on NixOS and it looks good to me. Ideally if anyone has access to a macOS machine to test on I'd love to get confirmation that it's working there as well before we merge.