Sidehistorik
I could not upgrade my work Confluence 7.19.X anymore, because the MySQL 8.x underneath was UTF-8 only.
So I found https://dba.stackexchange.com/questions/8239/how-to-easily-convert-utf8-tables-to-utf8mb4-in-mysql-5-5 - to whom the credit should go.
First - after stopping Confluence, snapshotting, backup etc:
...
Took 4-5 hours to complete, and should of cause end without errors....
Thats it - start Confluence and all is green .and it can be upgraded