Test external-static-plugins on recent GHCs

[hsyl20]

In https://github.com/input-output-hk/haskell.nix/blob/03c3581d2e0c91f7c2690115b487961ad62099a6/test/external-static-plugin/default.nix we have:

  meta.disabled = !(builtins.elem compiler-nix-name [
    "ghc810420210212"
  ]) || haskellLib.isCrossHost;

We should enable this test for GHC >= 9.6 as the patch (which was backported in ghc810420210212) is present.

[hsyl20]

Note that this is important in the context of the GHC 9.12 bump as GHC 9.12 alpha1 might have broken external static plugins. See discussion in https://gitlab.haskell.org/ghc/ghc/-/issues/25400#note_591724