Housekeeping for the gem: require Ruby3, updates to newest Rubocop with rubocop-rake and rubocop-rspec plugins

[simonneutert]

With this PR I intend to lay the groundwork for a 3.0 release of the gem.

As this PR will drop support for Ruby 2.x a major version bump is needed.

This PR does

• [x] bump the minimum required Ruby version to >= 3.0.0
• [x] updates the minimum bundler version accordingly
• [x] bumps the rubocop version
• [x] requires the rubocop-plugins rubocop-rake and rubocop-rspec and fixes what was off
• [x] updates the GitHub workflow run tests against the following ruby versions:
  • [x] mri ruby 3.0
  • [x] mri ruby 3.1
  • [x] mri ruby 3.2
  • [x] mri ruby 3.3
  • [x] jruby 9.4 🎁

---

closes https://github.com/dblock/strava-ruby-client/issues/70

[dblock]

I would not increment the version to 3.0.0 here because there's no API changes. If .gemspec is clear that Ruby >= 3.0 is required rubygems is just not going to let users on 2.7 upgrade to the new version anyway.