Deprecating legacy relying party

[Rod Widdowson]

As per the discussions yesterday, I have just added a warning when the parsers encounter a legacy <RelyingPatyGroup> element.  Case
1137.

I'm making a note of it here since that will give it a slightly wider airing - just in case there is more work we need to make sure
that the legacy syntax is hurried out. Like a release note.  I'll note that none of the other things we are deprecating (like the
legacy namespaces) got a release note except that we introduced the legacy warning in the same release that we introduced the new
namespace, whereas the new relying party syntax has been around is 3.0.

/Rod