ci: assigner: Use ubuntu-22.04 virtual environment

This commit updates the pull request assigner workflow to use the
Ubuntu 22.04 virtual environment.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>

.github/workflows/assigner.yml

@@ -15,7 +15,7 @@ jobs:
   assignment:
     name: Pull Request Assignment
     if: github.event.pull_request.draft == false
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-22.04
 
     steps:
     - name: Install Python dependencies