[AF] Extend enable-offer-store-unmasked-wallet-cards flag milestone

This flag uses a very old switch and will disappear in M78 without this
CL.  While I think we can likely get rid of this flag/switch and its
functionality, that won't be perfectly clear until the Derelict House
project fully launches right before the Autofill Auth project launches.
Extending this flag until that time comes.

Change-Id: I3ef6e1e7546dc59a6009179c936d6c9cd9e44ade
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1762656
Auto-Submit: Jared Saul <jsaul@google.com>
Commit-Queue: Siyu An <siyua@chromium.org>
Reviewed-by: Siyu An <siyua@chromium.org>
Cr-Commit-Position: refs/heads/master@{#688773}

chrome/browser/flag-metadata.json

@@ -1426,7 +1426,7 @@
   {
     "name": "enable-offer-store-unmasked-wallet-cards",
     "owners": [ "jsaul@google.com" ],
-    "expiry_milestone": 76
+    "expiry_milestone": 86
   },
   {
     "name": "enable-offline-previews",