Maintain Danger image with Node.js 14 etc.

Maintains the Danger image:

• Node.js: 12.4.0 => 14.16.1
• alex: 7 (7.1.0) => 9 (9.1.0)
• Ruby: 2.7.2 => 2.7.3
  • ruby:2.7-buster is now equivalent to ruby:2.7: prep for bullseye (Debian 11)
• deprecated link in comments

TODO

• Merge this MR
• Trigger a push job for :danger to push the image on the default branch after the merge

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Users of danger

• many repos in gitlab-org: https://sourcegraph.com/search?q=repo:gitlab.com/gitlab-org+registry.gitlab.com/gitlab-org/gitlab-build-images:danger&patternType=regexp
• gitlab-com/www-gitlab-com

Blocks !397 (merged) (#79)