Update flag expiration milestone for SplitSettingsSync

We're still targeting M82, but if it slips to M83 then we won't want
to clean up the flag until M84.

Bug: none
Change-Id: I5bf721beb684161ecc327e1f02cc95ba8ea9c0d8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2015671Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Commit-Queue: Kyle Horimoto <khorimoto@chromium.org>
Commit-Queue: James Cook <jamescook@chromium.org>
Auto-Submit: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#734288}

chrome/browser/flag-metadata.json

@@ -3360,7 +3360,7 @@
   {
     "name": "split-settings-sync",
     "owners": [ "jamescook", "cros-system-services@google.com" ],
-    "expiry_milestone": 82
+    "expiry_milestone": 84
   },
   {
     "name": "stop-in-background",