Swap out db-type and db-version for a more explicit db-image

[johnbillion]

Work in progress.

This swaps out db-type and db-version inputs for a more explicit db-image which contains values such as mysql:8.0 and mariadb:10.3.

The idea behind this is that we can be more explicit about differences between supported versions of MySQL and MariaDB without having to maintain confusing include and exclude rules that are hard to read. As someone who is not overly familiar with the available versions of each of the two, I find the matrices confusing. This should help a lot.

Trac ticket: https://core.trac.wordpress.org/ticket/63170

[github-actions]

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