Various changes relating to repo maintenance

[jorisdral]

We perform some maintenance on the repo. Amongst others:

• We remove the release process file. We also stop using scriv to manage changelogs. Since this repository does not see much traffic, it seems overkill to have a rigorous changelog policy.
• We update the .stylish-haskell.yaml configuration file to match the same file in consensus. This leads to some reformatting of imports in most/all Haskell modules.
• We copy two scripts from the fs-sim repository, one that formats cabal files using cabal-fmt, and one that performs cabal check on those files. We add the format-cabal.sh script as a CI check to the GitHub Actions workflow. We do not do this for the check-cabal.sh script.
• We make some update the remainder of the GitHub Actions workflow file.
• The CONTRIBUTING.md file no longer refers to the CONTRIBUTING.md file in the ouroboros-network repository.
• We remove the CHANGELOG.md file.