It is a pity that the requirement of MySQL 8 is rather hidden in the announcement and changelog for NC21. Actually, it would be interesting to know WHY MySQL 5.7 is not supported any more, i.e. which backward-incompatible changes have been introduced with NC21.
I have seen that quite a few NC users run their cloud on Plesk-hosted platforms provided through hosting providers. Since Plesk has only announced to support MySQL8 very recently, it may take still many months (if at all without a hosting package change) until these clients will have access to MySQL8. As a consequence, they are forced to stick to NC20 until that day.
Everything I see about plesk suggests that it supports mariadb, which is the “real” successor to mysql 5.x anyway (think of mysql 8 as oracle’s FORK of it – they stole the name by taking over Sun Microsystems, which held the name supposedly for safe keeping).
Dreamhost MySQL servers are largely all running version 8, so NC instances should be able to upgrade to version 21 of Nextcloud and work on their servers. It in fact DOES upgrade and work.
I think the original post’s issue is that NC on Dreamhost is incorrectly reporting that the MySQL version is “5.7.28-log”. This might be a bug unique to Dreamhost. Perhaps related to their slightly custom version of MySQL. I don’t know.
In terms of what workaround works on Dreamhost:
First, confirm your MySQL server is actually version 8. Then ignore the warning.
Still it would be good to pinpoint how this incorrect version number is being picked up by NC.