Add test matrix for multiple Ruby versions & update dependencies

[duffn]

This adds a test matrix for multiple versions of Ruby and grape-swagger.

I've tried to leave as much the same as I could, but some changes were necessary and I've also made a few guesses here:

• Support Ruby 2.7+
• Support grape-swagger >= 1.6.0
  • The old matrix had very old versions of the package, the most recent being from June 2019. I can add some more recent versions to the matrix if requested and can see what happens, but am not sure how many different versions we want to support.
• Support railties >= 6

Other notes

• Added inflections initializer for Failure/Error: mount API => '/'
• Sorted Gemfile
• Updated mime-types for error on Ruby 3 SyntaxError: /home/runner/work/grape-swagger-rails/grape-swagger-rails/vendor/bundle/ruby/3.0.0/gems/mime-types-2.99.3/lib/mime/types/logger.rb:26: _1 is reserved for numbered parameter
• Added webrick for Ruby 3
• Update rack to solve resolution conflicts
• gemspec rubocop formatting
• Update GitHub Actions versions

TODO

• Branched off of Migrate to GitHub Actions #105 so get that merged first
• Danger broke because I force pushed? Or because it's a fork? Danger fails on CI when running on PR from a fork in GH Actions / Buddy Build danger/danger#1103

[dblock]

Add fetch-depth like https://github.com/slack-ruby/slack-ruby-bot-server/blob/master/.github/workflows/danger.yml#L10, should fix danger.

[grape-bot]

	1 Message
📖	We really appreciate pull requests that demonstrate issues, even without a fix. That said, the next step is to try and fix the failing tests!

Generated by 🚫 Danger

[dblock]

Shall we increment the version to 0.4.0 and make a release? Want to help @duffn, remind me your Rubygems username?

[duffn]

@dblock Yeah, sounds good. It’s the same username.

[dblock]

Added you!

[duffn]

Accepted, thanks!