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

app/finders/user_finder.rb

@@ -53,6 +53,6 @@ def execute!
   end
 
   def input_is_id?
-    username_or_id.is_a?(Numeric) || username_or_id =~ /^\d+$/
+    @username_or_id.is_a?(Numeric) || @username_or_id =~ /^\d+$/
   end
 end