Make GPG validation case insensitive

What does this MR do?

Fixes validation of GPG signatures when the email address on the GPG key is not all in lower case.

Are there points in the code the reviewer needs to double check?

That I've correctly covered all cases.

Why was this MR needed?

Because GitLab forces addresses to lower-case (in my case, when getting the from LDAP), it means that all email address comparisons need to be case insensitive.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

#37009 (moved)