Upgrade to recent ubuntu image on circleci

CircleCI is retiring the ubuntu 14.whatever which is the default, and
is sending increasingly exasperated emails about it.

I borrowed the machine version from weaveworks/launcher#334 - there
may or may not be a newer version available, but using the same is
great for grepability.

.circleci/config.yml

@@ -1,6 +1,7 @@
 version: 2
 defaults: &defaults
   machine:
+    image: ubuntu-2004:202201-02
     docker_layer_caching: true
   working_directory: /home/circleci/src/github.com/weaveworks/service
   environment: