Change implementation of `FinalizeFecHeader` to write the FEC header
for multiple ssrcs according to the updated RFC.
Change-Id: I280964b2e53c3579f348fbd42815c966840375ac
Bug: webrtc:15002
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/307601
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Commit-Queue: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#40270}