[ci] Update CI to use Ubuntu 22.04 for testing

Author: kimihito

.github/workflows/ci.yml

@@ -14,7 +14,7 @@ jobs:
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:
-        os: [ubuntu-20.04]
+        os: [ubuntu-22.04]
         python-version:
             - "3.7"
             - "3.8"