[iOS] Extend default-browser-off-cycle-promo about flag

Extends the default-browser-off-cycle-promo chrome://flags entry
by several milestones. The feature is still needed and being experimented
with.

Bug: 448382772
Change-Id: Ibe9d2de0ad5dd3cf29d3aa5d29ad705cc430e217
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7499772
Reviewed-by: Alexandra Pereira <alexandrasp@google.com>
Commit-Queue: Guillaume Jenkins <gujen@google.com>
Reviewed-by: Ibrahim Kanouche <kanouche@google.com>
Reviewed-by: Xu Li <mxlg@google.com>
Cr-Commit-Position: refs/heads/main@{#1571840}