Keep the user and password definitions as e.g. dbconfig-common expects to
find them there. Extend the file to document (in context) why it is about
to be obsoleted to facilitate users migrating away from it.

Upstreamed from https://salsa.debian.org/mariadb-team/mariadb-10.5/-/commit/a6583c15226ad9a0ca37861e1bd01057bcff32a5