GHA: Get rid of Ubuntu 20.04

Author: vstinner

.github/workflows/build.yml

@@ -86,7 +86,7 @@ jobs:
 
           # Ubuntu: test deadsnakes Python versions which are not supported by
           # GHA python-versions.
-          - os: ubuntu-20.04
+          - os: ubuntu-22.04
             python: "3.6"
           - os: ubuntu-22.04
             python: "3.7"