[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

[jaymzh]

we shouldn't explicitly set dependencies of our dependencies, chef-zero should have this in it's deps. We should only list our direct deps.

[jaymzh]

this should be chef-workstation these days.

[dafyddcrosby]

I'd like to get away from chef-workstation as a dependency, since it's not entirely clear what's actually needed from chef-workstation. The chef-dk->chef-workstation transition was kind of awful (that's a beer conversation), and I'd like to avoid that again ;-).

That said, knife was moved workstation in Chef 17, so that should be explicitly specified, too. Will try to have those fixes up in the next few days

[jaymzh]

OK, so this is tricky.

TECHNICALLY GD, BM, and TT don't have a direct dependency on dk/ws.

However, they're generally intended to be installed as a devtools along side dk/ws. And we try to move more and more stuff into WS so we don't have to manage it in a fb-chefdk-gems sorta way. That's true, for example, with markdownlint, we got that into WS so we could stop managing it ourselves.

You're right though that strictly speaking, we don't need ws/dk, we just use those to provide our actual deps.