v2.8.0

Remove apt (ppa:hvr) fallback install method

This PR replicates haskell/actions#138 by Bodigrim.

For recent Ubuntu (> 20.04) the PPA hvr does not provide any GHC
releases, so this install method is now dead.

What's Changed

• Remove apt (ppa:hvr) fallback install method by @andreasabel in #121

Full Changelog: v2.7...v2.8.0