Document using MySQL with utf8mb4 by using docker-compose

utf8mb4 is already the default charset/collation for both MySQL and MariaDB in the docker-compose setup - it's set via --character-set-server=utf8mb4 / --collation-server=utf8mb4_bin in the command: section of the generated docker-compose.yml files. This has been the case since XDOCKER-285 (Nov 2023), which predates this issue. The downloadable docker-compose.yml files therefore already produce a utf8mb4 database out of the box, so no documentation change is needed.