Fix, Use devise_will_save_change_to_email? method for email change detection

[hnegishi]

Changes in this PR

This aligns with changes in devise.

• Change will_save_change_to_email? = false to devise_will_save_change_to_email? = false
• Remove email_changed? = false

Background

In the devise gem, the devise_will_save_change_to_email? method was introduced as a wrapper around will_save_change_to_email? or email_changed? to accommodate changes in dirty tracking. (Changed)

As a result, email validation in devise has also been updated to use devise_will_save_change_to_email?.

After that, only the devise_will_save_change_to_email? method, which wrapped email_changed?, was removed. Drop support to EOL rubies and Rails#5600.

For more details, please refer to the following devise Pull Request:
heartcombo/devise#5575
heartcombo/devise#5600

Effect

There are no behavioral changes introduced by this update.

• devise_token_auth does not use devise's email validation.
• Internally, devise_will_save_change_to_email? simply calls will_save_change_to_email?.

I would like to take this opportunity to thank the maintainers❤️