Extend form-controls-refresh expiry to M81.

This feature flag is still being used.

Bug: 959883
Change-Id: I7d1595454a271c6beba854f3a0565e40c1bc7928
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1857862Reviewed-by: Mason Freed <masonfreed@chromium.org>
Commit-Queue: Ionel Popescu <iopopesc@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#705606}

chrome/browser/flag-metadata.json

@@ -2198,7 +2198,7 @@
   {
     "name": "form-controls-refresh",
     "owners": [ "iopopesc@microsoft.com", "//third_party/blink/renderer/core/OWNERS" ],
-    "expiry_milestone": 78
+    "expiry_milestone": 81
   },
   {
     "name": "fractional-scroll-offsets",