* rm-old-base: remove ifdefs for pre-4.13 bases

4.13 and older have fallen out of the support window. Hence this commit
removes code only conditionally included for base 4.13 and older. Some
occasional transitive removals were implied and done in this same commit.

* Remove 8.6.5 from CI and Makefile

* Remove test for GHC <8.6.5

* Update GHC versions mentioned in the user guide

* rm-old-base: use Distribution.Compat.Prelude

The change was likely an artifact of a rebase.

* rm-old-base: restore builds not of cabal itself

The #ifdefs being generated need to be kept here so that projects other
than cabal can be built using older ghc versions and current cabal
versions.

* rm-old-base: restore older catchIO

This needs to be included so running with older bases and ghcs can be
done even while building cabal itself demands recent ghcs.

* Bump base lower bounds to >=4.13

* Update a few package version in the documentation

* rm-old-base: remove Distribution.Compat.Typeable

It's not needed with our currently supported ghcs.

* rm-old-base: restore T6906

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Co-authored-by: Andrea Bedini <andrea.bedini@tweag.io>
Co-authored-by: brandon s allbery kf8nh <allbery.b@gmail.com>
Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>