Check for tests against all R versions

[noamross]

As @Pakillo observed, we require testings against release, oldrel, and devel, (https://devguide.ropensci.org/ci.html#testing-using-different-versions-of-r), but this is not indicated in our automated checks. Is it possible to test if this is so in ci_results_gh(). I would think it would be challenging because there are many different ways a user might set up a matrix.

One possibility is to check if this is done in one of several expected ways (e.g., GH Actions YAML generated by usethis::use_github_action(), finding all three release, oldrel, and devel strings in the file, etc.), and otherwise note that it is indeterminate, with the report including a link to the CI folder in the repo to check manually.