Replace and ignore send_user_confirmation_email

What does this MR do and why?

This MR replaces the pre-existing send_user_confirmation_email application setting with the new email_confirmation_setting enum and ignores the old column.

This is done as a pre-requisite to remove the send_user_confirmation_email in milestone 15.8 and to remove soft_email_confirmation feature flag.

How to set up and validate locally

1. No references to send_user_confirmation_email other than the migrations from !102697 (merged) and ignore_column in ApplicationSetting model
2. If send_user_confirmation_email is set to true before db:migrate is run, then the new email_confirmation_setting should return hard
3. This can be confirmed by opening the rails console (gdk rails c) and verifying via Gitlab::CurrentSettings.email_confirmation_setting returning hard or Gitlab::CurrentSettings.email_confirmation_setting_hard? returning true

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #371638 (closed)