long tail of CI jobs

[flavorjones]

Split out from #2244, here's the long tail of lower-priority CI jobs and cI-related chores:

Build actions for:

• nightly builds against libxml2 master - build against upstream libxml2 and libxslt #2256
• get basic tests running: truffleruby - ci: add truffleruby to github actions #2270
• move generation of docker images to GA and use the Github Registry
• stop generating docker images for platforms no longer needed (like jruby)

Final steps:

• remove Concourse pipeline configs, rake config, and gem dependency - ci: drop concourse #2269
• remove any webhooks configured to ping concourse (e.g., PR or push)
• set required checks for merging PRs onto main
• update CONTRIBUTING.md section on CI - 909954b
• turn off code climate coverage, remove the badge from the readme - ci: drop concourse #2269

Note gaps relative to previous coverage:

• Installing gems on alpine/musl and on 32-bit linux (because actions/download-artifact won't run there, see Error: /etc/*release "no such file or directory" actions/checkout#334)

[flavorjones]

Remaining work pushed to #2271