No, at least not managed by Virtualmin. You could manually create the database user and permissions and point your apps to another version though. Same as if you were using some other unsupported database (e.g. mongo or whatever) with your apps. Webmin can support multiple MySQL installs using the module clone feature, so you can still manage it in a UI, but it’s not going to be something Virtualmin will be aware of…you’ll need to handle backups.
This is just such an unusual request (and a bad idea). It’s never come up.
CentOS has SCL versions of MySQL and MariaDB that can coexist with the standard package (because it installs into /opt, and can be configured separately with a little tweaking), but I don’t know about Ubuntu. This path is probably harder than just fixing the application to not use rely on old behavior. There are not a huge number of compatibility changes from 5.6 to 8.0, at least not in areas your apps care about (user management changed a lot, which caused us a lot of pain, but apps don’t care about how users are created and passwords are set or changed).
Install on an OS that has a version that suits your needs. Or, install the database before installing Virtualmin (but it needs to match the standard install paths, or it’ll prevent installation from working well). Much smarter just to use an OS that has the versions of stuff you need. It has other negatives, but if you need old software, just start with old software.