Bump expiry for dns-httpssvc flag to M91

Bug: 1052476
Change-Id: I0c8a0280fe121b82e17e602098732895ac9100f6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2507451Reviewed-by: Eric Orth <ericorth@chromium.org>
Commit-Queue: Dan McArdle <dmcardle@chromium.org>
Cr-Commit-Position: refs/heads/master@{#822172}

chrome/browser/flag-metadata.json

@@ -985,7 +985,7 @@
   {
     "name": "dns-httpssvc",
     "owners": [ "dmcardle", "ericorth" ],
-    "expiry_milestone": 88
+    "expiry_milestone": 91
   },
   {
     "name": "dns-over-https",