Update testing to Ubuntu Focal

Depends-On: https://review.opendev.org/c/zuul/zuul-jobs/+/831326
Change-Id: I5ca269f57866c4acf5c6e57c1146846e7cf50400

.zuul.yaml

@@ -26,7 +26,7 @@
       - python-builder-3.8-bullseye-container-image
       - python-base-3.8-bullseye-container-image
     provides: zuul-registry-container-image
-    nodeset: ubuntu-bionic
+    nodeset: ubuntu-focal
     vars: &image_vars
       docker_images:
         - context: .
@@ -44,7 +44,7 @@
     allowed-projects: zuul/zuul-registry
     run: playbooks/functional-test/run.yaml
     post-run: playbooks/functional-test/post.yaml
-    nodeset: ubuntu-bionic
+    nodeset: ubuntu-focal
     secrets:
       name: docker_credentials
       secret: zuul-registry-dockerhub