[usb] Bump expiration on new Windows backend flag

Work on this has not progressed but is still planned. Bumping expiration
to M-83.

Bug: 422562
Change-Id: Ie27a49e8893c39d40606e65f5823ec993eaefc73
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1759898
Commit-Queue: Vincent Scheib <scheib@chromium.org>
Reviewed-by: Vincent Scheib <scheib@chromium.org>
Cr-Commit-Position: refs/heads/master@{#688654}

chrome/browser/flag-metadata.json

@@ -2386,7 +2386,7 @@
   {
     "name": "new-usb-backend",
     "owners": [ "reillyg@chromium.org" ],
-    "expiry_milestone": 76
+    "expiry_milestone": 83
   },
   {
     "name": "newblue",