Defining a demotion process for a conformance test

[Clayton Coleman]

An issue was recently raised where a conformance test was demoted in the current release because we had wide agreement in sig-network that the behavior the test documented was too constraining to implementations.  However, we needed to backport that test demotion and it didn't happen and a conformance submission was blocked as a result.

We *have* backported fixes / demotions to tests in the past, but we didn't document it.  So I described a rough criteria here:

https://github.com/kubernetes/community/pull/7071

and would like feedback / additions / suggestions.  Hopefully matches everyone's understanding.