Drop support for older GHCs.

[phadej]

% cabal build -w  ghc-9.4.8 -c 'unix installed'
Resolving dependencies...
Error: [Cabal-7107]
Could not resolve dependencies:
[__0] trying: tar-0.7.2.0 (user goal)
[__1] trying: file-io-0.2.0 (dependency of tar)
[__2] next goal: unix (dependency of file-io)
[__2] rejecting: unix-2.7.3/installed-2.7.3 (conflict: file-io => unix>=2.8.0.0 && <3)

and cabal.haskell-ci in the repository indeed says

installed: -directory -unix -bytestring -filepath -text -process -binary

But honestly, if I'm stuck on older GHCs and updating tar would ask me to upgrade bytestring, i'd just NOT update tar (and first work on updating GHC). In other words, tar doesn't have enough weight to justify updating boot libs.