Drop Focal integration testing

There are projects which no longer support python3.8 in our integration
test. This means they cannot be installed on Ubuntu Focal any longer
without extra steps like checking out old eol tags. Rather than try and
keep up with this list (that will only grow over time) we drop the Focal
integration test and simply rely on unittesting for python3.8 and older.

This is in line with previous decisions made for python2.7 testing. It
isn't ideal, but should still provide sufficient coverage for the most
glaring incompatibilities we may try to introduce.

Change-Id: I7c80e93627597101cbed64a0efdc18140dbc854a

.zuul.yaml

@@ -99,14 +99,6 @@
     vars:
       pbr_pip_version: 'git+https://github.com/pypa/pip.git#egg=pip'
 
-- job:
-    name: pbr-installation-openstack-focal
-    parent: pbr-installation-openstack
-    nodeset: ubuntu-focal
-    description: |
-      Run pbr integration testing on older Ubuntu which allows us
-      to test compatibility with older versions of pip and setuptools.
-
 - project:
     templates:
       - lib-forward-testing-python3
@@ -124,7 +116,6 @@
         - openstack-tox-py310
         - openstack-tox-py311
         - pbr-installation-openstack
-        - pbr-installation-openstack-focal
         - pbr-installation-openstack-pip-dev
     gate:
       jobs:
@@ -138,7 +129,6 @@
         - openstack-tox-py310
         - openstack-tox-py311
         - pbr-installation-openstack
-        - pbr-installation-openstack-focal
         - pbr-installation-openstack-pip-dev
     periodic:
       jobs: