Add support for Ruby 3.0

[denzelem]

• Add Ruby 3.0 to the test matrix
• Run the gem with Ruby 2.7 and check for deprecation warnings. You might want to check out https://makandracards.com/makandra/496481-changes-to-positional-and-keyword-args-in-ruby-3-0 as help to fixing them.
• Try out the gem in a fresh Rails 6.1 project with Ruby 3.0