Drop support for Ubuntu Focal

Its Python 3.8 is not supported by any active version of Ansible.
Focal has been best-effort for a while already.

Change-Id: Id04f9b9835cc705716a797caa7beef2536027186

doc/source/install/index.rst

@@ -22,10 +22,6 @@ Full support (fully tested in the CI, no known or potential issues):
 * Ubuntu 22.04 "Jammy"
 * Debian 11 "Bullseye" and 12 "Bookworm"
 
-Limited support, may be removed at any moment:
-
-* Ubuntu 20.04 "Focal"
-
 .. note::
    Operating systems evolve and so does the support for them, even on stable
    branches.

releasenotes/notes/no-focal-28675cbdf1b7b9f6.yaml

@@ -0,0 +1,5 @@
+---
+upgrade:
+  - |
+    Ubuntu 20.04 "Focal" is no longer a supported operating system, please
+    use 22.04 "Jammy" instead.

zuul.d/bifrost-jobs.yaml

@@ -78,25 +78,21 @@
     parent: bifrost-integration-tinyipa
     nodeset: ubuntu-bionic
 
-# NOTE(rpittau): used by openstack/ironic, openstack/ansible-collections-openstack,
-# openstack/ironic-inspector, openstack/openstacksdk
+# NOTE(rpittau): used by openstack/ansible-collections-openstack, openstack/openstacksdk
+# https://review.opendev.org/c/openstack/ansible-collections-openstack/+/903956
+# https://review.opendev.org/c/openstack/openstacksdk/+/903973
 - job:
     name: bifrost-integration-tinyipa-ubuntu-focal
     parent: bifrost-integration-tinyipa
     nodeset: ubuntu-focal
 
-# NOTE(rpittau): used by openstack/ansible-collections-openstack
-- job:
-    name: bifrost-integration-tinyipa-keystone-ubuntu-focal
-    parent: bifrost-integration-tinyipa
-    nodeset: ubuntu-focal
-
 - job:
     name: bifrost-integration-tinyipa-ubuntu-jammy
     parent: bifrost-integration-tinyipa
     nodeset: ubuntu-jammy
 
-# NOTE(rpittau): remove after antelope
+# NOTE(rpittau): used by openstack/ansible-collections-openstack
+# https://review.opendev.org/c/openstack/ansible-collections-openstack/+/903956
 - job:
     name: bifrost-integration-tinyipa-keystone-ubuntu-focal
     parent: bifrost-integration-tinyipa

zuul.d/project.yaml

@@ -19,8 +19,6 @@
         - bifrost-upgrade-ubuntu-jammy
         - bifrost-upgrade-keystone-centos9
         # Non-voting jobs
-        - bifrost-integration-tinyipa-keystone-ubuntu-focal:
-            voting: false
         - bifrost-integration-redfish-pxe-uefi-centos-9:
             voting: false
         - bifrost-integration-dhcp-ubuntu-jammy: