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/issuable_finder.rb

@@ -254,7 +254,7 @@ def assignee
       if assignee_id?
         User.find_by(id: params[:assignee_id])
       elsif assignee_username?
-        UserFinder.new(params[:assignee_username]).find_by_username
+        User.find_by_username(params[:assignee_username])
       else
         nil
       end
@@ -282,7 +282,7 @@ def author
       if author_id?
         User.find_by(id: params[:author_id])
       elsif author_username?
-        UserFinder.new(params[:author_username]).find_by_username
+        User.find_by_username(params[:author_username])
       else
         nil
       end