Add MySQL 9.1, MariaDB 11.4, and other matrix adjustments

[desrosj]

This adds the latest MySQL innovation release to the testing matrix: 9.1. It also adds the newest LTS version of MariaDB (11.4).

Additionally, it makes some adjustments to the MariaDB matrix to improve the overall coverage a bit.

Technically, back to MariaDB 5.5 is still supported by WordPress, even though <= 10.4 has reached EoL. However, 5.5 is equivalent to MySQL 5.5, which is not currently tested due to the fact that the Docker containers are not compatible with modern machines and do not start.

With a small change to the docker-compose.yml file, the MariaDB 5.5 containers do start successfully. Testing against this version essentially gives us MySQL 5.5 testing. The matrix has been adjusted to include all MariaDB versions with greater than 1% of usage with WordPress in the wild according to the wordpress.org Stats page and the latest LTS version (currently 11.4).

Trac ticket: https://core.trac.wordpress.org/ticket/62221

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