Add support for Ruby 3

[lowang]

Testing:

• switch to ruby 3
• rspec

[connorshea]

Out of curiosity, why is Ruby 3 noted as only being supported on Rails 6.1?

[rabotyaga]

Out of curiosity, why is Ruby 3 noted as only being supported on Rails 6.1?

actually it should be >= 6.1, and this is only because rails prior to 6.1 doesn't support ruby >= 3.0.0

[alishersadikov]

Out of curiosity, why is Ruby 3 noted as only being supported on Rails 6.1?

actually it should be >= 6.1, and this is only because rails prior to 6.1 doesn't support ruby >= 3.0.0

I am not sure this is accurate any more - maybe it was at the time of this post? Rails 6.0.1 seems to support Ruby 3:
https://rubygems.org/gems/rails/versions/6.0.1

Also here:
https://www.fastruby.io/blog/ruby/rails/versions/compatibility-table.html

Not sure if you meant the recommended Ruby version is 3 on Rails 6.1.