Update duet flag expiration

Change-Id: I936f2388c769ccec2d5ed0ae32d893cd3c1670da
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1917881
Auto-Submit: Matthew Jones <mdjones@chromium.org>
Reviewed-by: Theresa  <twellington@chromium.org>
Commit-Queue: Theresa  <twellington@chromium.org>
Cr-Commit-Position: refs/heads/master@{#715927}

chrome/browser/flag-metadata.json

@@ -1123,7 +1123,7 @@
   {
     "name": "enable-chrome-duet",
     "owners": [ "mdjones" ],
-    "expiry_milestone": 79
+    "expiry_milestone": 82
   },
   {
     "name": "enable-chrome-duet-labels",