Drop support for ruby 2.7

[Taeir]

Ruby 2.7 has been EOL for a while now, and the Codidact production instances have been moved over to Ruby 3. I think it is about time to drop ruby 2.7 support. This would allow us to clean up a bit, and avoid some questions:

• Remove CI tests for ruby 2.7 (less duplication in file)
• Change gemfile ruby version (should fix rvm automatic version switching)
• Delete now unnecessary dependencies (fixes ruby 3.0/3.1 version conflicts due to older version in gemfile than in base ruby)
• Ability to update packages to newer versions that have dropped ruby 2.7 support.
• Update rubocop to 3.0 syntax