Extend expiry milestone for secure DNS flags.

We will continue to run our secure DNS auto-upgrade experiment past
M81, so the ability to opt-out via chrome://flags is still needed.

Bug: 985589
Change-Id: I82f38a153815817a2dccfec2c697be56678ff88f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2023286Reviewed-by: Eric Orth <ericorth@chromium.org>
Commit-Queue: Katharine Daly <dalyk@google.com>
Cr-Commit-Position: refs/heads/master@{#735536}

chrome/browser/flag-metadata.json

@@ -844,7 +844,7 @@
   {
     "name": "dns-over-https",
     "owners": [ "dalyk", "doh-core@google.com" ],
-    "expiry_milestone": 81
+    "expiry_milestone": 84
   },
   {
     "name": "document-passive-event-listeners",