Make getting a user by the username case insensitive

What does this MR do?

This merge request makes sure that when creating a new user object searching by username, it does so in a case sensitive manor.

What are the relevant issue numbers?

Closes #38304 (closed)
Closes #27229 (closed)
Possibly solves #47601 (moved)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated (After searching, nothing in the docs suggest usernames are sensitive or insensitive),
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides

doc/api/README.md

@@ -233,7 +233,10 @@ provided you are authenticated as an administrator with an OAuth or Personal Acc
 
 You need to pass the `sudo` parameter either via query string or a header with an ID/username of
 the user you want to perform the operation as. If passed as a header, the
-header name must be `Sudo`.
+header name must be `Sudo`. 
+
+NOTE: **Note:**
+Usernames are case insensitive.
 
 If a non administrative access token is provided, an error message will
 be returned with status code `403`: