Remove contact as a supported object type in EPP (#2954)

This primarily affects the EPP greeting. We already were erroring out when any
contact flows attempted to be run; this should just prevent registrars from even
trying them at all.

This PR is designed to be minimally invasive, and does not remove any of the
contact flows or Jakarta XML/XJC objects/files themselves. That can be done
later as a follow-up.

Also note that the contact namespace urn:ietf:params:xml:ns:contact-1.0 is still
present for now in RDE exports, but I'll remove that subsequently as well.

This is a redo of PR #2932, which had been reverted, but now controlled via
FeatureFlag so that it won't be enabled until we schedule it to do so (and only
after sufficient time has passed after notifying registrars in advance).

BUG= http://b/475506288

load-testing/src/main/java/google/registry/client/resources/login.xml

@@ -11,7 +11,6 @@
     <svcs>
       <objURI>urn:ietf:params:xml:ns:host-1.0</objURI>
       <objURI>urn:ietf:params:xml:ns:domain-1.0</objURI>
-      <objURI>urn:ietf:params:xml:ns:contact-1.0</objURI>
     </svcs>
   </login>
   <clTRID>epp-client-login-@@NOW@@-@@CHANNEL_NUMBER@@</clTRID>