drops refs to bionic and replace with focal

.ci/scripts/test_old_deps.sh

@@ -1,6 +1,6 @@
 #!/usr/bin/env bash
 
-# this script is run by GitHub Actions in a plain `bionic` container; it installs the
+# this script is run by GitHub Actions in a plain `focal` container; it installs the
 # minimal requirements for tox and hands over to the py3-old tox environment.
 
 set -ex

.github/workflows/tests.yml

@@ -141,7 +141,7 @@ jobs:
     steps:
       - uses: actions/checkout@v2
       - name: Test with old deps
-        uses: docker://ubuntu:bionic # For old python and sqlite
+        uses: docker://ubuntu:focal # For old python and sqlite
         with:
           workdir: /github/workspace
           entrypoint: .ci/scripts/test_old_deps.sh
@@ -213,15 +213,15 @@ jobs:
       fail-fast: false
       matrix:
         include:
-          - sytest-tag: bionic
+          - sytest-tag: focal
 
-          - sytest-tag: bionic
+          - sytest-tag: focal
             postgres: postgres
 
           - sytest-tag: testing
             postgres: postgres
 
-          - sytest-tag: bionic
+          - sytest-tag: focal
             postgres: multi-postgres
             workers: workers