Launchpad charm build fails because of invalid charmcraft.yaml config

This fix moves the charm build recipes to use charmcraft 2.x/stable
instead of older versions of charmcraft.
We are experiencing build failures because charmcraft thinks
otherwise valid charmcraft.yaml configuration is invalid.

Closes: Issue openstack-charmers#28

charmed_openstack_info/data/lp-builder-config/ovn.yaml

@@ -4,7 +4,7 @@ defaults:
   branches:
     master:
       build-channels:
-        charmcraft: "2.1/stable"
+        charmcraft: "2.x/stable"
       channels:
         - latest/edge
       bases: