ci: stop using deprecated Ubuntu 18.04

GitHub Actions fails running 18.04 jobs currently:
<https://github.com/actions/runner-images/issues/6002>

.github/workflows/jsonrpc-client-npm-package.yml

@@ -9,7 +9,7 @@ on:
 jobs:
   pack-module:
     name: "Package @deltachat/jsonrpc-client and upload to download.delta.chat"
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     steps:
       - name: Install tree
         run: sudo apt install tree