docs: recommend Ubuntu 20.04 instead of Bionic
docs: recommend Ubuntu 20.04 instead of Bionic

Bionic support was dropped in I39e38e4a6c2e52dd3822c9fdea354258359a9f53.

Change-Id: I765aac352590fd2f74d3fd90676d6d098548e6b8

openstack-dev/devstack,openstack-dev/devstack,nawawi/openstack,nawawi/openstack