More precise description of, and CI for, OS support tiers

Today, the platform support tier structure is less specific than it probably should be. It lists "macOS" and "Linux" rather than e.g. "Windows 11", "macOS Ventura", or "Fedora 37".

It would be useful to have a more specific policy - clearly we don't expect GHC to work on Windows 98, on Mac OS X 10.4, or on every experimental, obscure Linux distro. The table should list specific distributions as well as specify versions (e.g. "releases from the last five years" or "last three major releases").

Additionally, CI for bindists should be set up to ensure that reality and the policy continue to match one another by testing the bundled binaries on fresh images for each supported system.