ci: delete ci/ (#22227)

Having CI scripts that is separate from the build system causes
tremendous amounts of problems, headaches and bugs. Testing the validity
of the scripts locally become near impossible as time goes on as they're
only vetted if it works on whatever CI provider we happened to have at
the time, with their own quirks and behavior.

The extra indirection between "cmake <-> general CI scripts <-> GHA" is
also a frequent source of problems, as the orchestration needs to be
done with environment variables, cmake flags and github actions matrix
strategy. This combination has turned out to be exceptionally fragile.

Examples:
https://github.com/neovim/neovim/commit/15394b6855c3b17be06bf2bfbac7797d9c3ebf1d
https://github.com/neovim/neovim/commit/13aa23b62af4df3e7f10687b76fe8c04efa2a598
https://github.com/neovim/neovim/pull/22072#discussion_r1094390713

A lot of the code was inlined to .github/workflows/ci.yml without
further modifications. While this in itself doesn't integrate with our
build system any more than the current situation, it does
1. remove a level of indirection, and more importantly
2. allow us to slowly start integrating the CI into our build system now
   that all the relevant code is in one place.

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