Use GHA, set minimal Ruby to 2.7, update dependencies

[ofedoren]

WDYT @ekohl and @adamruzicka ? I'd like to drop ancient relics, since the only usage of this lib known to me/rubygems is hammer-cli, which already requires Ruby 2.7 at least.

[ofedoren]

Thanks, @ekohl ! I've dropped rest-client complexity and using shared workflow now. Not sure why it didn't trigger unlike the previous one...

[ofedoren]

Thanks, @ekohl, this indeed makes more sense, updated.

[ekohl]

GitHub doesn't trigger unknown actions. What I usually do is configure it to run on push as well (also has the benefit that it can cache gemsets between runs on different PR authors) and then push the branch temporarily to upstream. Once CI runs, I delete the branch again. I think actions are recognized by name.

[ofedoren]

Thanks, @ekohl ! Seems green, worse case: we'll catch issues in hammer.