Use newer OS images for train and ussuri LB jobs

Diskimage-builder now requires at least python 3.8.

Unfortunately the amphora image building in octavia's devstack plugin
doesn't respect the python version set via devstack's PYTHON3_VERSION
environment variable. Instead, let's switch to Ubunto 20.04 that ships
with python 3.8 as the default python interpreter.

openstack/diskimage-builder@fe0e532

.github/workflows/functional-loadbalancer.yaml

@@ -28,10 +28,10 @@ jobs:
             ubuntu_version: "20.04"
           - name: "ussuri"
             openstack_version: "stable/ussuri"
-            ubuntu_version: "18.04"
+            ubuntu_version: "20.04"
           - name: "train"
             openstack_version: "stable/train"
-            ubuntu_version: "18.04"
+            ubuntu_version: "20.04"
     runs-on: ubuntu-${{ matrix.ubuntu_version }}
     name: Deploy OpenStack ${{ matrix.name }} with Octavia and run loadbalancer acceptance tests
     steps: