CI: migrate away from Ubuntu 20.04.

Author: jwilk

.github/workflows/ci.yml

@@ -60,14 +60,10 @@ jobs:
     strategy:
       matrix:
         include:
-        - python-version: '3.6'
-          os: ubuntu-20.04
-        - python-version: '3.7'
-          os: ubuntu-20.04
         - python-version: '3.8'
-          os: ubuntu-20.04
+          os: ubuntu-22.04
         - python-version: '3.9'
-          os: ubuntu-20.04
+          os: ubuntu-22.04
         - python-version: '3.10'
           os: ubuntu-22.04
         - python-version: '3.11'