templates: update Usernetes to gen2-v20231218.0

Signed-off-by: Akihiro Suda <[email protected]>

examples/experimental/u7s.yaml

@@ -52,7 +52,7 @@ provision:
     set -eux -o pipefail
     test -d ~/usernetes && exit 0
     cd ~
-    git clone --branch=gen2-v20230919.0 https://github.com/rootless-containers/usernetes
+    git clone --branch=gen2-v20231218.0 https://github.com/rootless-containers/usernetes
 - mode: user
   script: |
     #!/bin/bash