Update ci.yml ubuntu versions

ubuntu 18.04 eol 06/2023
use ubuntu 20.04.
ubuntu-latest is 22.04 now.
see https://docs.github.com/en/actions/using-github-hosted-runners/about-github-hosted-runners#supported-runners-and-hardware-resources

.github/workflows/ci.yml

@@ -22,7 +22,7 @@ jobs:
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:
-        os: [ubuntu-18.04, ubuntu-latest, macos-latest, windows-latest]
+        os: [ubuntu-20.04, ubuntu-latest, macos-latest, windows-latest]
         python: ["3.8", "3.9", "3.10"]
 
     steps: