Push referrer granularity flag expiry to M81

We want to keep this around for a couple more milestones
because it will be a way to test/opt out of the new referrer
policy default (crbug.com/1014207).

Bug: 1014207
Change-Id: I67fe1b41ecf2c1ca76b744f4c1dcd1c7da2b8f02
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1865557
Auto-Submit: David Van Cleve <davidvc@chromium.org>
Reviewed-by: Lily Chen <chlily@chromium.org>
Commit-Queue: Lily Chen <chlily@chromium.org>
Cr-Commit-Position: refs/heads/master@{#706674}

chrome/browser/flag-metadata.json

@@ -2944,8 +2944,8 @@
   },
   {
     "name": "reduced-referrer-granularity",
-    "owners": [ "jochen", "mkwst" ],
-    "expiry_milestone": 78
+    "owners": [ "davidvc", "mkwst", "jochen" ],
+    "expiry_milestone": 81
   },
   {
     "name": "release-notes",