[debian-mysql] Renaming mysql->mariadb, maintainer script refactoring, Shellcheck etc

[Otto Kekäläinen]

Hi Tuukka and Daniel!

I noticed upstream commits about the things mentioned in the email
subject. I opened this MR to apply the same changes in Debian, and
while I was reviewing them I noticed that the same changes can be
applied in even more files, so I went through all of `debian/`
categorically and filed
https://salsa.debian.org/mariadb-team/mariadb-server/-/merge_requests/49.

Reviews welcome!

Once merged and tested in Debian unstable for a while the same changes
can be upstreamed back into 11.2.

- Otto