Cleanup ruby usage in workflows and gemspec dependency versioning

[jonabc]

This PR is meant to improve the general hygiene of this repo relating to actions workflows and gem requirements

1. Updates actions workflows to remove specific caching of gems installed by script/bootstrap, and instead use the built-in caching mechanism from ruby/setup-ruby
   • script/boostrap is no longer used in CI
2. Updates actions workflows to use the ruby version from .ruby-version instead of manually setting a version in every job'
3. Updates .ruby-version to 3.1.3, the latest stable release, and updates the tested versions of ruby and bundler in CI
   • Licensed no longer needs to target ruby 2.6 now that exes aren't packaged and included in releases
4. Bumps the gem's required ruby version to >= 2.6 to remove the explicit dependency on bundler
   • bundler is included in ruby 2.6+ at versions that licensed supports
5. Sets specific dependency versions in the gemspec file which should trigger dependabot to keep dependencies up to date

[jonabc]

The licensed-ci GitHub Action has created a pull request containing license metadata updates based on the changes in this branch.

Please review the pull request for any additional changes required and merge when ready.