[CrOS Settings] Extend split-settings-sync flag to M-92

Feature has not launched yet, so pushing flag expiration back a few
milestones.

Change-Id: I982ad888d985c5ae798d2039defdea8fe68840a3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2505796
Commit-Queue: Kyle Horimoto <khorimoto@chromium.org>
Commit-Queue: James Cook <jamescook@chromium.org>
Auto-Submit: Kyle Horimoto <khorimoto@chromium.org>
Reviewed-by: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#821890}

chrome/browser/flag-metadata.json

@@ -4400,7 +4400,7 @@
   {
     "name": "split-settings-sync",
     "owners": [ "jamescook", "cros-system-services@google.com" ],
-    "expiry_milestone": 87
+    "expiry_milestone": 92
   },
   {
     "name": "ssl-committed-interstitials",