Use existing-user language in reconfirmation email

What does this MR do?

To resolve a security issue, we sent re-confirmation emails to many existing users of gitlab.com. Unfortunately, most of those emails said "Thanks for signing up to GitLab!" That spooked users, because they had not just signed up (example).

This causes existing-user language to be used in Devise confirmation emails when the user appears to be in need of re-confirmation. See https://gitlab.com/gitlab-org/gitlab/-/issues/220756#note_377459554.

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 #219558