Extend account-id-migration flag to M88

The account ID migration launch was delayed as it considered a relatively
risky rollout until after the release process is stabilised.

Bug: None
Change-Id: I1cec176237c49d3139914e97f1f4d181deed6b7f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2174506
Reviewed-by: David Roger <droger@chromium.org>
Commit-Queue: Mihai Sardarescu <msarda@chromium.org>
Cr-Commit-Position: refs/heads/master@{#765089}

chrome/browser/flag-metadata.json

@@ -36,7 +36,7 @@
   {
     "name": "account-id-migration",
     "owners": [ "msarda" ],
-    "expiry_milestone": 84
+    "expiry_milestone": 88
   },
   {
     "name": "aggregated-ml-app-ranking",