Use Buildkite to test JuliaSyntax against older Julia versions

[topolarity]

This PR moves the JuliaSyntax GHA matrix (from JuliaLang/julia#59870) to Buildkite.

(big thanks to @DilumAluthge for most of the work here)

[DilumAluthge]

Todo list:

• Make sure all JuliaSyntax jobs are green.
• Compare the matrix here against the GHA matrix, and make sure no jobs are missing. (We have to disable the Windows Julia 1.0 jobs for now until we fix the S3 aliases.)
• Uncomment all the lines here that we previously commented out.
• Get a full green CI build here.
• Change the Julia branch being used here from the caf/... branch to master, and make sure that:
  • The JuliaSyntax jobs go away.
  • Everything else stays green here.
• Merge this PR.
• Rebase the upstream PR (Move JuliaSyntax + JuliaLowering into the main tree JuliaLang/julia#59870), and make sure that all the JuliaSyntax Buildkite jobs in the matrix correctly launched and passed on that PR.
• Remove all GitHub Actions files from the upstream PR (Move JuliaSyntax + JuliaLowering into the main tree JuliaLang/julia#59870)

[topolarity]

• Get a full green CI build here.

Looks green to me!

doctest failure (https://buildkite.com/julialang/julia-buildkite/builds/2298#019a75fd-9ddb-4ee5-b4f9-aa698dd32268/753-857) is expected due to running on a branch. Other failures seem to match what's failing on https://buildkite.com/julialang/julia-master/builds/51976 (inherited from master at time of PR).

Time to remove the branch and then merge if everything is green.

[c42f]

Awesome, thanks so much!