mariadb-storage-rocksdb DEB package installs rocksdb.cnf in wrong directory

The mariadb-plugin-rocksdb pacakge for Ubuntu 20.04 installs an extra rocksdb.cnf containing plugin-load-add=ha_rocksdb.so, but it always puts it into /etc/mysql/mariadb.conf.d/. Config files from this directory are only included by default starting with MariaDB 10.5 though, so for 10.4 and older the file should be installed in /etc/mysql/conf.d/ instead.