php8.0-mbstring : Depends: libonig5 (>= 6.8.1) but it is not installable code example

Example: como instalar ext-mbstring en ubuntu 18.04

deb http://archive.ubuntu.com/ubuntu bionic main multiverse restricted universe
deb http://archive.ubuntu.com/ubuntu bionic-security main multiverse restricted universe
deb http://archive.ubuntu.com/ubuntu bionic-updates main multiverse restricted universe