s4:kdc: Replace FAST cookie with dummy string

s4:kdc: Replace FAST cookie with dummy string

All that uses the FAST cookie is the gss-preauth authentication
mechanism, which is untested in Samba, and disabled by default.
Disabling the FAST cookie code (and sending a dummy string instead)
relieves us of the maintenance and testing burden of this untested code.

Signed-off-by: Joseph Sutton <josephsutton@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Stefan Metzmacher <metze@samba.org>
Autobuild-User(master): Stefan Metzmacher <metze@samba.org>
Autobuild-Date(master): Wed Jun 21 13:19:17 UTC 2023 on atb-devel-224