Drop support ruby 2.7 and active_support 6.1

This PR officially drops support for Ruby 2.7.0 and active_support 6.1

Changes

• The gemspec already requires Ruby >= 3.0
• CI workflows are already configured to test only Ruby 3.0 and newer versions
• This aligns with the project's forward-looking approach and simplifies maintenance

Related Changes

• Update Rubocop's target to 3.0 and fix cops

Testing

• CI will run against all supported Ruby versions (3.0+)
• No additional testing needed as this is a support declaration change

Documentation

• No documentation changes needed as README already states Ruby 3.0+ requirement