Stop using ubuntu 20.04

See actions/runner-images#11101

Author: cushon

.github/workflows/ci.yml

@@ -80,7 +80,7 @@ jobs:
         # Use "oldest" available ubuntu-* instead of -latest,
         # see https://docs.github.com/en/actions/using-github-hosted-runners/about-github-hosted-runners/about-github-hosted-runners#standard-github-hosted-runners-for-public-repositories;
         # due to https://github.com/google/google-java-format/issues/1072.
-        os: [ubuntu-20.04, macos-latest, windows-latest]
+        os: [ubuntu-22.04, macos-latest, windows-latest]
     runs-on: ${{ matrix.os }}
     continue-on-error: true
     steps: