Drop support for older GHCs.

```
% 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.

Start with cabal.haskell-ci and inspect the installed-package constraints shown in the issue, then trace which older GHC configurations the repository still supports. Confirm the intended support boundary before changing the configuration. Done means the older GHC support is removed without requiring the incompatible boot-library combination described in the report.

Written by the indexing model from the issue text.