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/models/user.rb

@@ -259,7 +259,7 @@ def inactive_message
   scope :order_recent_sign_in, -> { reorder(Gitlab::Database.nulls_last_order('current_sign_in_at', 'DESC')) }
   scope :order_oldest_sign_in, -> { reorder(Gitlab::Database.nulls_last_order('current_sign_in_at', 'ASC')) }
   scope :confirmed, -> { where.not(confirmed_at: nil) }
-  scope :by_username, -> (usernames) { iwhere(username: usernames) }
+  scope :by_username, -> (usernames) { iwhere(username: Array(usernames).map(&:to_s)) }
 
   # Limits the users to those that have TODOs, optionally in the given state.
   #