MAINTAINERS: dccp: move Gerrit Renker to CREDITS

As far as I can tell we haven't heard from Gerrit for roughly
5 years now. DCCP patch would really benefit from some review.
Gerrit was the last maintainer so mark this entry as orphaned.

Subsystem DCCP PROTOCOL
  Changes 38 / 166 (22%)
  (No activity)
  Top reviewers:
    [6]: kstewart@linuxfoundation.org
    [6]: allison@lohutok.net
    [5]: edumazet@google.com
  INACTIVE MAINTAINER Gerrit Renker <gerrit@erg.abdn.ac.uk>

Signed-off-by: Jakub Kicinski <kuba@kernel.org>