Moving generator method to private

[kekke-n]

Hi.

I noticed that there is refactoring point in lib/generators/active_record/devise_generator.rb, and so I created PR.

All public methods in devise_generator.rb is called by executing rails g active_record:devise. But I think that those methods except for copy_devise_migration, generate_model and inject_devise_content are not needed to call because of not commands which generate files.

I think it is better that those methods are defined as private.

After changing code, I checked generator command and minitest, then it seems no problem as before.

exec rails g active_record:devise hoge

 rails g active_record:devise hoge
      create  db/migrate/20220115061612_devise_create_hoges.rb
      create  app/models/hoge.rb
      invoke  rspec
      create    spec/models/hoge_spec.rb
      insert  app/models/hoge.rb

exec bin/test test/generators/active_record_generator_test.rb

 bin/test test/generators/active_record_generator_test.rb  
==> Devise.orm = :active_record
Run options: --seed 40973

# Running:

..........

Finished in 0.071228s, 140.3942 runs/s, 687.9317 assertions/s.
10 runs, 49 assertions, 0 failures, 0 errors, 0 skips

Feel free to close if it's best left as-is.