Big Bang Bot: Update CODEOWNERS

Action	User	Reason
Remove	@mark.nelson	Username is not present in either team or contributor whitelist.
Add	@peter.sigur	User added from team charter mission team manifest
Add	@amitg	User added from team charter mission team manifest
Add	@blairbowden	User added from team charter mission team manifest
Add	@piotr.machaj	User added from team charter mission team manifest
Add	@steven.donald	User added from team charter mission team manifest
Add	@bjacksonfv	User added from team charter mission team manifest
Add	@kliu	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.