[github] Remove Ruby 2.5+2,6 tests, add Ruby 3.0+3.1 tests, update dev gems

[dafyddcrosby]

Several smaller commits in one PR (check the commits tab for blow-by-blow), setting the stage for getting this stuff on modern versions

• Ignore local copy of rubocop config
• Bump actions/checkout to get rid of CI errors
• Bump rubocop to same version as facebook/chef-cookbooks
• Remove chef-dk (it's super dead now)
• Explicitly use chef-zero >=15, since it's needed for Ruby 3