Big Bang Bot: Update CODEOWNERS

Action	User	Reason
Remove	@montgomery.marcus	Employment dates no longer active
Remove	@charden	Employment dates no longer active
Add	@sarnowski-unicorn	User added from team charter mission team manifest
Add	@kylegalloway-seed	User added from team charter mission team manifest

These changes were automatically generated based on the state stored in the team charter mission team manifest. If these changes are not expected, please refer to updating team charter manifest.