CI against Ruby 3.2, 3.3, Drop Ruby 2.7 and Rails 6.0

[krororo]

Add CI against Ruby 3.2 and 3.3. Drop support Ruby 2.7 and Rails 6.0.

The combination of Ruby 3.2+ and Rails 6.1 was excluded due to the problem of not being able to use keyword arguments for scopes.
see: rails/rails#46934

Ruby 2.7 and Rails 6.0 were dropped because they are EOL.

• https://www.ruby-lang.org/en/news/2023/03/30/ruby-2-7-8-released/
• https://rubyonrails.org/2023/6/26/Rails-Versions-7-0-5-1-6-1-7-4-have-been-released

Code no longer needed due to dropping Rails 6.0 will be removed in another PR.

[Dooor]

🚀 🚀 🚀

[krororo]

Thanks!