[Sync] Mark deprecated GetUpdatesOrigin::RETRY as reserved

Followup to https://crrev.com/c/6576860 which deprecated this entry,
but didn't mark it as reserved.

Proto cleanup only, no behavior change.

NO_IFTTT=No actual enum change, just marking values as reserved.

Bug: 40252048
Change-Id: I7a89a98e4a8450874225757f06f07ddbe6e5f48a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6610963
Reviewed-by: Jood Hajeer <jood@google.com>
Auto-Submit: Marc Treib <treib@chromium.org>
Commit-Queue: Jood Hajeer <jood@google.com>
Cr-Commit-Position: refs/heads/main@{#1468148}