This certificate update does not have anything to do with your app itself. It relates to the push server that is actually sending the push notifications.
If you have your own server, then you have to install the new root certificate. If you are using a 3rd party service to send push notifications, you may want to contact them and make sure they are complying with this change.
As for the question of protocol, if you are not using the HTTP/2 protocol do send push notifications you should start doing so ASAP, as the old protocol will stop working on March 31st, 2021. If you are not sure which protocol you are using, you will want to check with your server team, or if you are using a 3rd party service, contact them and confirm.
