WIP: MDEV-26850: Never delete my.cnf, always use update-alternatives instead

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In Debian the file /etc/my.cnf is supposed to point to mariadb.cnf. This symlink is created and maintained by update-alternatives as run from mysql-common (the real Debian package) or a tweaked version of mariadb-common from mariadb.org repositories.

Do not just bluntly replace it with a link during upgrades.

This fixes a regression introduced in 07d1c8567cbfe which lacks a commit message explaining why this file was added in the first place.