[MariaDB developers] More extensive major version upgrade testing

Hi, If any dev here is interested in the topic of ensuring the major version upgrades (and essentially running mariadb-upgrade) continues to work flawlessly and regressions are less likely to surface unnoticed, please review and consider adding your approvals to https://github.com/MariaDB/server/pull/3503 and share feedback on WIP https://github.com/MariaDB/server/pull/3563. Thanks!