[RSearches] Extend the flag to M89.

The Related Searches flag has expired, and we want it to stay active.
This extends it until M89.
BUG=1069096

Change-Id: Ib437a8f7b00833516be113d9a2216a0f9fa72880
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2412830
Auto-Submit: Donn Denman <donnd@chromium.org>
Reviewed-by: Jinsuk Kim <jinsukkim@chromium.org>
Commit-Queue: Donn Denman <donnd@chromium.org>
Cr-Commit-Position: refs/heads/master@{#810074}

chrome/browser/flag-metadata.json

@@ -3853,7 +3853,7 @@
   {
     "name": "related-searches",
     "owners": [ "ayman", "donnd" ],
-    "expiry_milestone": 85
+    "expiry_milestone": 89
   },
   {
     "name": "release-notes-notification",