Switch from CircleCI to GitHub Actions

[mattbrictson]

Capistrano currently uses CircleCI to test against Ruby versions going all the way back to 2.0.

Maintaining the CI configuration has become more of a burden due to our reliance on the official Docker images for Ruby. The Docker images for old versions of Ruby are based on very outdated versions of Ubuntu that are hard to support. This has resulted in a number of CI failures that have to be regularly fixed: #2130, #2124, #2116, #2106.

GitHub Actions seems like it would be easier to maintain. SSHKit has already been migrated to GitHub Actions. I would like to do the same for Capistrano.