ci: report coverage to codeclimate

[nadiamoe]

This PR enables reporting coverage data to CodeClimate through GHA.

Additionally, I have remove the coverage.xml file which I don't think that should have been commited in the first place.

Gotchas

• coverageLocations should not be needed, but a bug in the codeclimate action breaks reporting if ommited. A fix for that bug is already submitted upstream and hopefully will land soon.
  • Issue closed upstream, workaround removed
• CC_TEST_REPORTER_ID does not need to kept secret. Making it public has some pros and cons:
  • ➖ Ill-intended agents could upload fake coverage reports to codeclimate, which could potentially deface PR stats. However, codeclimate prevents multiple reports for a single commit, so a potential malicious agent would need to be faster uploading the fake report for a PR than the CI is. I deem that unlikely, or at least too costly for a prankster.
  • ➕ We save from adding more secrets to the repo
  • ➕➕ We are able to see coverage diff for external PRs