Remove support for GHC8.10

[Cmdv]

We are dropping support for GHC8.10 so we need to remove all references of it from our repo.

[rdlrt]

Just a note to ensure ghc9* CI builds work prior to dropping ghc8.10, as per current latest commit - they dont:

[Cmdv]

There is going to be a slight delay on this as we can only go ahead one cardano-node has done this work too. By that time I'd imagine we'll have the CI failures fixed, from my understanding it's nix related and is being worked on.

[sgillespie]

Just a note to ensure ghc9* CI builds work prior to dropping ghc8.10, as per current latest commit - they dont

Unfortunately the GitHub actions are in a pretty bad state right now, but we DO have working builds in our public Hydra: https://ci.iog.io/eval/62899

[rdlrt]

Unfortunately the GitHub actions are in a pretty bad state right now

Afaik - hydra CI is based on nix, the github CI is our only point of reference for working cabal builds