attempt to use local actions

[geekosaur]

This PR redesigns our PR CI:

• It moves a number of things to GitHub composite actions so they can be reused
• It runs more things in parallel, reducing CI from nearly 3 hours in some cases to under an hour
• It centralizes things like validated and bootstrapped GHC versions in a config.yml file rather than scattering them throughout the codebase

It also paves the way for things like

• validating FreeBSD, static Alpine, and Intel Macs
• doing prereleases from a dedicated job instead of trying to do them from CI (which won't work if CI doesn't get run as part of merging)

(Note that, as part of the second point above, this rewrite doesn't currently do prereleases at all. I can re-add that until said dedicated job is deployed.)

More details are in .github/README.md, which additionally details various other parts of our CI infrastructure.

Template B: This PR does not modify behaviour or interface

E.g. the PR only touches documentation or tests, does refactorings, etc.

Include the following checklist in your PR:

• Patches conform to the coding conventions.
• Is this a PR that fixes CI? If so, it will need to be backported to older cabal release branches (ask maintainers for directions). (No point in backporting, as even the LTS prerelease part has to live on master.)

[geekosaur]

okay, took a bit of fighting only to discover that a couple of typoes led to empty strings instead of useful errors, but I've merged the validate and bootstrap lists (see comments in .github/config.yml).

[geekosaur]

Next push will have a preliminary changelog. Would still like feedback on documentation, but I suspect we need to get 3.16 out of the way first.

[geekosaur]

I ended up switching the changelog to a README.md because this only affects CI and changelog-d doesn't like changelog fragments that don't list any packages.

Possible change I'm considering: someone has a yq analogous to jq, available for install via an action. I may look up how to install it separately, because it's the Makefile that really needs it so the ugly and fragile sed for make bootstrap-jsons can be replaced with a proper query of config.yml.

Other considerations:

• location and naming of aforementioned README.md, which isn't really suitable for either end users or most contributors, only for people intending to work on CI
• location of config.yml, with many of the same considerations

[geekosaur]

Bloody caches…

[geekosaur]

…right, and now it runs. Still, will need to switch to artifacts to avoid stuff randomly failing because GitHub ran their cache cleanup.

[geekosaur]

Welp. I researched how to save just the testing artifacts, got suspicious about test-runtime-deps, and promptly ran into #11048 which means that everything must be cached for the whole thing to work. We get away with it currently because build and test is all the same job, ignoring validate-old-ghcs which has to rebuild everything for the extra-ghc anyway. Which means there's no way to run tests in parallel without running afoul of GitHub periodically purging caches.