Add Big Sur (Intel) to Dorico System Requirements

I saw John Barron announced that Dorico 3.5 supports macOS Big Sur:
https://www.facebook.com/groups/dorico/permalink/2794364864174581/

However, this page is still out-of-date:
https://new.steinberg.net/system-requirements/

Yes, we know. It will be updated in due course.