Move lower constrants job to ubuntu focal

As per the lates testing runtime the minimum
python version and distro we are using is py38 and ubuntu focal.
So moving lower constraints job to those for testing the
lower bound of defined testing runtime.

Change-Id: Ifbffa10ba2346959869eb819162453f64e1a0540

zuul.d/jobs.yaml

@@ -877,7 +877,7 @@
 - job:
     name: openstack-tox-lower-constraints
     parent: openstack-tox
-    nodeset: ubuntu-bionic
+    nodeset: ubuntu-focal
     branches: ^(?!driverfixes/).*$
     timeout: 2400
     description: |
@@ -889,7 +889,7 @@
     irrelevant-files: *common-irrelevant-files
     vars:
       tox_envlist: lower-constraints
-      bindep_profile: test py36
+      bindep_profile: test py38
 
 - job:
     name: build-openstack-puppet-tarball