Merge deprecate-jelly-instance-support-7653

Author: wolfgang61, exarkun
Reviewer: exarkun
Fixes: #7653

Formally deprecate the `instance` atom in the Jelly protocol.
Twisted has not emitted this token for at least ten years.


git-svn-id: svn://svn.twistedmatrix.com/svn/Twisted/trunk@43253 bbbe8e31-12d6-0310-92fd-ac37d47ddeeb