CI: Update Ubuntu versions

Ubuntu 22.04 images are now available on GitHub Actions. Add 22.04 and
drop 18.04, as we are dropping 18.04 for an upcoming release.

.github/workflows/main.yml

@@ -201,7 +201,7 @@ jobs:
     runs-on: ${{ matrix.ubuntu }}
     strategy:
       matrix:
-        ubuntu: ['ubuntu-20.04', 'ubuntu-18.04']
+        ubuntu: ['ubuntu-20.04', 'ubuntu-22.04']
     if: always()
     needs: [clang_check]
     defaults: