dcsctp: Resend SHUTDOWN ACK if receiving SHUTDOWN

According to RFC 9260 Section 9.2, once an endpoint has reached the
SHUTDOWN-RECEIVED state, it MUST continue responding to SHUTDOWN chunks
from its peer.

Previously, HandleShutdown would explicitly return without action if the
state was SHUTDOWN-ACK-SENT. This caused the socket to ignore the peer's
retransmitted SHUTDOWN chunks (sent if our first SHUTDOWN ACK was lost),
leading to a delay until the socket's own timer expired to resend
SHUTDOWN ACK.

This CL fixes this by calling SendShutdownAck() when a SHUTDOWN chunk is
received in the SHUTDOWN-ACK-SENT state, which resends the ACK and
restarts the T2-shutdown timer.

Bug: webrtc:500087337
Change-Id: I4d35156b02aad529bcd68906d9d0c8898b53eb6a
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/462001
Commit-Queue: Victor Boivie <boivie@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#47368}