Update ubuntu nodesets to focal

The openstack-single-node nodeset still is using ubuntu-bionic,
which is nearly 4 years old now. This change updates it to use
the newer ubuntu focal release.

Depends-on: https://review.opendev.org/c/openstack/openstack-helm-infra/+/839996
Depends-on: https://review.opendev.org/c/openstack/openstack-helm-infra/+/840370

Change-Id: Ia43cb31e13bc059541116064aa2092526186b831

zuul.d/jobs.yaml

@@ -16,7 +16,7 @@
 - job:
     name: openstack-helm-lint
     run: playbooks/lint.yml
-    nodeset: ubuntu-bionic
+    nodeset: ubuntu-focal
     # NOTE(aostapenko) Required if job is run against another project
     required-projects:
       - openstack/openstack-helm-infra

zuul.d/nodesets.yaml

@@ -15,7 +15,7 @@
     name: openstack-helm-single-node
     nodes:
       - name: primary
-        label: ubuntu-bionic
+        label: ubuntu-focal
     groups:
       - name: primary
         nodes:
@@ -25,11 +25,11 @@
     name: openstack-helm-ubuntu
     nodes:
       - name: primary
-        label: ubuntu-bionic
+        label: ubuntu-focal
       - name: node-1
-        label: ubuntu-bionic
+        label: ubuntu-focal
       - name: node-2
-        label: ubuntu-bionic
+        label: ubuntu-focal
     groups:
       - name: primary
         nodes: