Let dh_systemd handle most of the systemd side and
get rid of custom scripts

Rework installation of systemd service and socket files
base on Michael Biebl merge request:

https://salsa.debian.org/mariadb-team/mariadb-server/-/merge_requests/63
https://salsa.debian.org/mariadb-team/mariadb-server/-/merge_requests/75