SUPER users created before 10.11 should retain READ_ONLY ADMIN privilege upon upgrade

MDEV-29596 removed READ_ONLY ADMIN privilege from the SUPER alias, so new users which are granted SUPER privilege won't get READ_ONLY ADMIN automatically. However, to maintain backward compatibility, the old users (created before the change) which have SUPER privilege shouldn't lose the READ_ONLY ADMIN capacity, so READ_ONLY ADMIN privilege should be granted to them upon upgrade.