Update expiry milestone for printer settings UI overhaul

- Updated expiry milestone for printer settings UI overhaul from M79 to
  M81.

Bug: 1005905
Change-Id: Id8e6649a74d10ea8698a9116431ef50d208ab6fc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1853145
Commit-Queue: Bailey Berro <baileyberro@chromium.org>
Reviewed-by: Bailey Berro <baileyberro@chromium.org>
Cr-Commit-Position: refs/heads/master@{#704682}

chrome/browser/flag-metadata.json

@@ -1141,7 +1141,7 @@
   {
     "name": "enable-cups-printers-ui-overhaul",
     "owners": [ "jimmyxgong" ],
-    "expiry_milestone": 79
+    "expiry_milestone": 81
   },
   {
     "name": "enable-custom-mac-paper-sizes",