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/controllers/profiles/keys_controller.rb

@@ -36,7 +36,7 @@ def destroy
   def get_keys
     if params[:username].present?
       begin
-        user = User.find_by_username(params[:username])
+        user = UserFinder.new(params[:username]).find_by_username
         if user.present?
           render text: user.all_ssh_keys.join("\n"), content_type: "text/plain"
         else