Clarify which macOS are supported.

The sad truth is that we can easily support only the few latest macOS version. I.e. 10.15 at the moment.
- GitHub CI: https://help.github.com/en/actions/reference/software-installed-on-github-hosted-runners has macOS 10.15
- Travis CI has bigger list: https://docs.travis-ci.com/user/reference/osx/ but the timelimit on the jobs makes it quite unusable (we were barely able to get `cabal-install` built and tested there).

Relatedly, it's also not-so-clear which is the oldest macOS supported by GHC official bindists (cc @bgamari). Our `CONTRIBUTING.md` says that we support `cabal-install` to be buildable with three years old GHC. So that three-years-oldest GHC's oldest supported macOS sets the ultimate hard-limit.

It would be good to write this down, and see what can be done to actually test on these older configurations. The recent #6833 shows that some people are stuck on them, for whatever reason.

Also local development machines are tricky. I have a 10.12 machine, which I don't upgrade primarily because it could be used to debug stuff. I'm lucky I don't have any other real use for that machine.

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Same applies for Windows, yet Windows 10 is continuously rolling release, which is a curse and a blessing. Should we test with Windows 2016 server edition, as GitHub CI generously offers that option too? Notice however, that the oldest GHC on windows we currently support is GHC-8.6.5 (which is just a *one year old*). This is very unfortunate.