gitlab-ci: update CI configuration

Ben Boeckelrequested to merge
ben.boeckel/rust-yaml-merge-keys:ci-cleanups into master

Major changes:

  • split into separate files
  • move from YAML anchors to extends: syntax
  • use rules rather than only
  • shorten up job names and move stage bits to be suffixes

Also fixes minvers and adds audit jobs.

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