GHCup metadata

Adding a new GHC version

1. open the latest ghcup-<yaml-ver>.yaml
2. find the latest ghc version (in yaml tree e.g. ghcupDownloads -> GHC -> 8.10.7)
3. copy-paste it
4. adjust the version, tags, changelog, source url
5. adjust the various bindist urls (make sure to also change the yaml anchors)
6. run cabal run ghcup-gen -- check -f ghcup-<yaml-ver>.yaml
7. run cabal run ghcup-gen -- check-tarballs -f ghcup-<yaml-ver>.yaml -u 'ghc-8\.10\.8'
8. run cabal run ghcup-gen -- generate-hls-ghcs -f ghcup-<yaml-ver>.yaml --format json -o hls-metadata-0.0.1.json
9. run cabal run ghcup-gen -- generate-table -f ghcup-<yaml-ver>.yaml --stdout and adjust docs/install tables