NC Upgrade from 20 to 21. Mimium PHP version supported?

Running NC 20.0.8
Rasperian OS on RaspPi4 4Gb
Php v7.3.27-1~deb10u1

I would appreciate clarification on the minimum php version required for upgrade to NC 21. I currently run v7.3.27. Can I upgrade to NC 21 or does the upgrade first require me to upgrade php to v8?

Thank you!

Do you that a Nextcloud administrator guide exists, which contains many useful information, including system requirements etc.?!