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

lib/api/users.rb

@@ -186,6 +186,7 @@ def reorder_users(users)
         optional :username, type: String, desc: 'The username of the user'
         use :optional_attributes
       end
+      # rubocop: disable CodeReuse/ActiveRecord
       put ":id" do
         authenticated_as_admin!
 
@@ -224,6 +225,7 @@ def reorder_users(users)
           render_validation_error!(user)
         end
       end
+      # rubocop: enable CodeReuse/ActiveRecord
 
       desc 'Add an SSH key to a specified user. Available only for admins.' do
         success Entities::SSHKey