Resolve "contributor emails in contributors graphs is case sensitive"

What does this MR do?

changes author email to a case insensitive comparison so emails like "Somebody@foo.com" and "somebody@foo.com" are treated as a single author in contribution graphs.

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

should be pretty simple

Why was this MR needed?

see above description

Screenshots (if relevant)

before:

after:

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #19550 (closed)