Extend expiry of experimental-productivity-features flag

Update the expiry to M86. We will move the related features to use Document Policy, meaning developers will then need time to experiment/test the policies locally.

Bug: 906163
Change-Id: I6d6d524d21e0d177956badb240312ac572f408cc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2015488Reviewed-by: Ian Clelland <iclelland@chromium.org>
Commit-Queue: Jason Chase <chasej@chromium.org>
Cr-Commit-Position: refs/heads/master@{#734322}

chrome/browser/flag-metadata.json

@@ -1362,7 +1362,7 @@
   {
     "name": "enable-experimental-productivity-features",
     "owners": [ "feature-control@chromium.org" ],
-    "expiry_milestone": 81
+    "expiry_milestone": 86
   },
   {
     "name": "enable-experimental-web-platform-features",