🔧(circle) update image tag using default

Circle has removed several images and the added a tag named default to
avoid having to update the image name in the future.

Author: lunika

.circleci/config.yml

@@ -6,7 +6,7 @@ aliases:
   - &defaults
     # We use the machine executor, i.e. a VM, not a container
     machine:
-      image: ubuntu-2004:2022.07.1
+      image: default
       # Prevent cache-related issues
       docker_layer_caching: false
     working_directory: ~/fun
@@ -684,7 +684,7 @@ jobs:
   hub:
     # We use the machine executor, i.e. a VM, not a container
     machine:
-      image: ubuntu-2004:202201-02
+      image: default
       # Prevent cache-related issues
       docker_layer_caching: false