ACTION-623: Schema update for SerialNumber

All,
This is in response to my ACTION-623 that I got a couple of weeks ago.

After some investigation, we'd be concerned about doing this change in the existing XML Signature 1.0 namespace. While Microsoft implementations should be able to handle such an update, as noted earlier  by Pratik there are in general no guarantees for it and in addition it seems that this was precisely the reason namespaces were introduced.

Doing it in the 1.1 (and 2.0) namespace however seems appropriate.

-- Magnus