Add a warning for enum elements with 'not_' prefix.

[edudepetris]

Summary

Addresses #36272

I added a new warning when an user creates enum elements with not_ prefix

Example

class Foo < ActiveRecord::Base
  enum status: [:sent, :not_sent]
end

$ bin/rails c 
Loading development environment (Rails 6.1.0.alpha)
 :001 > Foo
An enum element in Foo uses the prefix 'not_'. This will cause a conflict with auto generated negative scopes.

The enum element not_sent trigger the new warning

Other Information

CHANGELOG updated.