allow building with Stackage lts-21

[juhp]

Fixes #6159

I tested by building locally with ghc-9.4 stackage nightly (before lts-21.0 was released)

[mpilgrem]

@juhp, thanks. The linting is failing because of the duplicated LANGUAGE CPP in Stack.Docker. I think it does not build on Windows because there is no macro MIN_VERSION_unix(0,7,0) because there is no unix package; that macro may need to be guarded with more CPP to test for non-Windows.

I am not sure what is causing the Linux integration tests to fail so early. I've re-run it, in case it was a glitch somewhere. EDIT: That job passed when re-run, so it must have been a glitch.

[mpilgrem]

On unix-compat, I think the solution is to back-port to Stack 2.9.3.1 the fix in 6c85911. EDIT: The background to that commit is at: #6069.

[mpilgrem]

I have also updated the stack.yaml and the helpers for people who use Cabal (the tool). The former will cause the CI to fail - it fails with GHC 9.4.5 in any event, because of the problems identified at #6161.

[juhp]

Thanks for all the fixups

[juhp]

Just noting that stack-2.9.3.1 is finally in lts-21.3 yay!

Thanks again