Declare required_ruby_version 2.7 and run CI against Ruby 2.7 - 3.1, and minimum Rubocop version

[sambostock]

The last time the config was updated (#386), we missed that we were adding a dependency on a more recent version of Rubocop than the one declared as minimum in the gemspec, as mentioned in #388 (comment).

To address this, and prevent it from happening again, the following changes are made:

• the minimum rubocop version is bumped accordingly
• in addition to the version in Gemfile.lock, CI now also runs against the version of rubocop declared to be the minimum

This more recent version also drops support for Ruby 2.5 and made me realize we should be testing against multiple Ruby versions, so the following other changes are made:

• required_ruby_version is declared in gemspec
  • 2.7 was chosen as it is the oldest version not at end-of-life
• CI now runs against Rubies 2.7, 3.0, and 3.1
  • a test is added to ensure this list always includes the required_ruby_version

[volmer]

Sorry I'm late to this but if we always want to use the minimum required version of RuboCop specified on the gemspec for testing then why don't we just declare it directly on the Gemfile?