Extend smart-dim-model-v3 flag

Hoping to experiment with this flag outside dev/canary from M80.

Bug: no
Change-Id: I9e105127e942e9ab39d07683c89ebb545cb645b0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1926093
Auto-Submit: Andrew Moylan <amoylan@chromium.org>
Reviewed-by: Jia Meng <jiameng@chromium.org>
Commit-Queue: Andrew Moylan <amoylan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#717478}

chrome/browser/flag-metadata.json

@@ -3304,7 +3304,7 @@
   {
     "name": "smart-dim-model-v3",
     "owners": [ "amoylan", "jiameng" ],
-    "expiry_milestone": 80
+    "expiry_milestone": 83
   },
   {
     "name": "smart-text-selection",