Use Ubuntu 22.04 instead of 20.04.

Author: samsrabin

.github/workflows/test-doc-builder.yml

@@ -17,7 +17,7 @@ jobs:
 
   test-python-oldest:
     name: Test in Python ${{ matrix.python-version }}
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-22.04
     strategy:
       fail-fast: false
       matrix: