Unexpire caBLEv2 flag.

This flag expired in M88. It's taking longer than hoped, but this is
still a going concern so unexpire for another six months.

BUG=1002262

Change-Id: I4c8e98e393bd0f30eece8f5065fe574cf47fedb0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2495706
Commit-Queue: Adam Langley <agl@chromium.org>
Commit-Queue: Martin Kreichgauer <martinkr@google.com>
Auto-Submit: Adam Langley <agl@chromium.org>
Reviewed-by: Martin Kreichgauer <martinkr@google.com>
Cr-Commit-Position: refs/heads/master@{#820357}

chrome/browser/flag-metadata.json

@@ -2296,7 +2296,7 @@
   {
     "name": "enable-web-authentication-cable-v2-support",
     "owners": [ "webauthn-team@google.com" ],
-    "expiry_milestone": 87
+    "expiry_milestone": 91
   },
   {
     "name": "enable-web-authentication-chromeos-authenticator",