This dependency is only used to import `coerce`, but it's also
available from `base`, so we can easily stop depending on it
and update its upper bound whenever it's updated.

Problem: there are 4 major GHC versions newer than 8.4, so we can freely
disregard versions older than 8.6. But we still have CPP conditional
checking that GHC version is 8.6 or newer.

Solution: drop that CPP check.
Also require `base >= 4.12` since it's the version bundled with 8.6.1
and it seems to be the standard approach.

https://hackage.haskell.org/package/doctest-0.20.0/changelog
Tests pass with 0.20.

Changelog: https://hackage.haskell.org/package/hedgehog-1.1/changelog

That's a temporary change, we are waiting for `gauge` to
support latest GHC.

Problem: there is the following change in GHC-9.2.1:
> GHCi’s :kind! command now expands through type synonyms
> in addition to type families.

We have some doctests where we expect things like `18 :% 11` in
output of `:kind!`, but `:%` is a type synonym.
Those tests pass before GHC-9.2.1 because type synonym is not
expanded, but they fail starting from GHC-9.2.1 because
`:%` is expanded into `'::%`.

Solution: use CPP to expect different results depending on GHC version.