Merge lp:~paride/britney/mysql-8 into lp:~ubuntu-release/britney/hints-ubuntu

My second take at this :)

This change will allow mysql-8.0/8.0.20-0ubuntu1 to migrate from groovy-proposed. According to
LP: #1877504 there is an actual regression in MySQL 8.0.20 and a proper patch is being worked on, however we are in a situation where:

 * MySQL 8.0.20 already made it into Focal via the security pocket as version mysql-8.0/8.0.20-0ubuntu0.20.04.1, uploaded to fix the CVEs mentioned in the changelog, therefore:
 * The regression already hit Focal, where we mostly want to avoid regressions, and:
 * Groovy has unfixed CVEs, as the regression was caught by a failing autopkgtest on libdbd-mariadb-perl.
 * A proper fix is being worked on, but we can't know how much time will be needed to get it right and validate it.
 * The regression is hit only when using mariadb, which is from universe, while mysql-8.0 is in main.

In my opinion migrating mysql-8.0/8.0.20-0ubuntu1 and having the CVEs fixed is more important than avoiding a regression that already hit Focal.

Ok, after reading the bug history, I don't think that this should be ignored in order to let this package through. This is a serious regression in an SRU which needs to be fixed, and there's no reason to force it through in devel (which should not be used in production, so CVEs are not as severe) as opposed to focusing on a proper fix for the regression.