Adopt new GHC support policy

The README claimed a minimum supported version of GHC 8.4.x, but that no
longer matched reality. While fixing the GHC < 9.8 build issues in #781
I checked which versions xmobar actually builds with, and the oldest
that succeeds is 9.2:

  - GHC 8.4/8.6 fail because colour-2.3.6 requires base >= 4.13 (i.e.
    GHC >= 8.8).
  - GHC 8.8/8.10/9.0 fail because Xmobar.Plugins.Accordion uses GHC APIs
    they lack (withFileBlocking, hGetContents').

Rather than add compatibility shims to keep claiming support for
versions upstream itself no longer recommends, #784 settled on a
simpler, sustainable policy: officially support only the GHC versions
upstream marks as `Suitable for use` in the GHC status overview
(https://gitlab.haskell.org/ghc/ghc/-/wikis/GHC-Status). At the time of
writing that means 9.6 through 9.14.

This commit implements that policy:

  - Bump the lower bound on base to >= 4.18 (GHC 9.6) so the build
    constraints match the documented support, instead of silently
    failing on older compilers.
  - Document the policy and the current 9.6-9.14 range in the README,
    replacing the stale 8.4.x note.

Users who need older GHC versions can continue to use older xmobar
releases.

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