Push back expiry of avif-gainmap-hdr-images flag.

The flag should stay around until the ISO spec is finalized.

Bug: 360924314

Change-Id: I3708f636e903521865cc582b1ed43dc8e5d90cbd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5823924


Reviewed-by: James Zern <jzern@google.com>
Commit-Queue: Maryla Ustarroz-Calonge <maryla@google.com>
Reviewed-by: Wan-Teh Chang <wtc@google.com>
Cr-Commit-Position: refs/heads/main@{#1349310}