WIP: Unable to register user with duplicate unconfirmed email

What does this MR do?

There is an email uniqueness constraint that prevents duplicate email addresses. The problem is that this constraint does not incorporate email confirmation. Anyone can add any email address to their account and prevent the legitimate owner of the email address from registering with GitLab.

This MR will only check existing confirmed email addresses when validating email uniqueness.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Closes #25571