The "$mysql_statedir/debian-$MAJOR_VER.flag" is not used by any
maintainer script ("$mysql_datadir/debian-$MAJOR_VER.flag" is used,
https://github.com/MariaDB/server/blob/10.6/debian/mariadb-server-10.6.postinst#L164).

See: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=985870

Fix also some minor typo.