Switch away from ubuntu-latest (#8606)

I noticed warnings on Azure like [this](https://dev.azure.com/certbot/certbot/_build/results?buildId=3311&view=logs&j=d74e04fe-9740-597d-e9fa-1d0400037dfd) which say:

> ##[warning]Ubuntu-latest pipelines will use Ubuntu-20.04 soon. For more details, see https://github.com/actions/virtual-environments/issues/1816

I was worried about us suddenly switching to Ubuntu 20.04 and things breaking so I tested that `ubuntu-20.04` works and am opening this PR to switch things over explicitly now. I'd rater have our VM images pinned to specific versions than a generic version specification like `latest` which might see an upgrade and break our tests unexpectedly.

I ran the notification code on Ubuntu 20.04 at https://dev.azure.com/certbot/certbot/_build/results?buildId=3315&view=results and you can see the notification at https://opensource.eff.org/eff-open-source/pl/ojjhde5j4jyw7dcurd5zfduymr.

.azure-pipelines/templates/jobs/standard-tests-jobs.yml

@@ -73,6 +73,6 @@ jobs:
       - template: ../steps/tox-steps.yml
   - job: test_sphinx_builds
     pool:
-      vmImage: ubuntu-latest
+      vmImage: ubuntu-20.04
     steps:
       - template: ../steps/sphinx-steps.yml

.azure-pipelines/templates/stages/notify-failure-stage.yml

@@ -5,7 +5,7 @@ stages:
         variables:
           - group: certbot-common
         pool:
-          vmImage: ubuntu-latest
+          vmImage: ubuntu-20.04
         steps:
           - bash: |
               set -e