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

@@ -35,7 +35,7 @@ def find_by_username!
 
   # Tries to find a User by username or id, returning nil if none could be found.
   def execute
-    if input_is_id?(@username_or_id)
+    if input_is_id?
       find_by_id
     else
       find_by_username
@@ -45,14 +45,14 @@ def execute
   # Tries to find a User by username or id, raising a `ActiveRecord::RecordNotFound` if it could
   # not be found.
   def execute!
-    if input_is_id?(@username_or_id)
+    if input_is_id?
       find_by_id!
     else
       find_by_username!
     end
   end
 
-  def input_is_id?(input)
-    input.is_a?(Numeric) || input =~ /^\d+$/
+  def input_is_id?
+    @username_or_id.is_a?(Numeric) || @username_or_id =~ /^\d+$/
   end
 end