HaTS Next: Update demo flag expiry

Extend the expiry of the flag used to force a HaTS Survey for demo/test
purposes.

Bug: 1110888
Change-Id: I4e7d02d6ea30a449707ff862b388c0d0bf09dd57
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2351829Reviewed-by: Martin Šrámek <msramek@chromium.org>
Commit-Queue: Theodore Olsauskas-Warren <sauski@google.com>
Cr-Commit-Position: refs/heads/master@{#797188}

chrome/browser/flag-metadata.json

@@ -2673,8 +2673,7 @@
     "owners": [ "//chrome/browser/ui/hats/OWNERS" ],
     // A debugging and demo flag to allow UI/dev/testing team to always show the UI
     // components for the survey without being limited by the triggering mechanism.
-    // This flag should expire along with the above feature flag.
-    "expiry_milestone": 84
+    "expiry_milestone": 90
   },
   {
     "name": "happiness-tracking-surveys-for-desktop-migration",