[rubygems/rubygems] Introduce the Gem::CIDetector

This is based on the list in Gem::UpdateSuggestion and Bundler::Fetcher;
these have similar purposes (determining whether/what CI we're executing
in), and can benefit from being combined and updated (they're both
slightly out of date).

Noteable changes:
* We'll consider ourselves to be on a CI in more cases - if CI_NAME or
  TASKCLUSTER_ROOT_URL are set specifically, since those represent two
  cases that were either overlooked or are no longer covered by the
  existing implementation. (Or possibly TaskCluster still does provide
  RUN_ID, but failed to document it)
* We will notice/track a few additional services in ci_strings (cirrus,
  dsari, taskcluster), stop tracking 'snap' (they went under in 2017),
  and update buildbox to buildkite (they've been called that for 8
  years, and the BUILDBOX envs have been deprecated for 3).
* We'll also sort/uniq/downcase the values (all of which only matter
  because of the special case of CI_NAME).

https://github.com/rubygems/rubygems/commit/60652b942f

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