Extend expiry of sync-support-secondary-account flag to M80

This flag should get removed soon (see bug), but let's keep it around
until that actually happens.

Bug: 990745
Change-Id: Ibaf96e78ebcb959ae6bcc44d71e3debced233405
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1760964
Commit-Queue: Marc Treib <treib@chromium.org>
Commit-Queue: Jan Krcal <jkrcal@chromium.org>
Auto-Submit: Marc Treib <treib@chromium.org>
Reviewed-by: Jan Krcal <jkrcal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#688455}

chrome/browser/flag-metadata.json

@@ -2998,7 +2998,7 @@
   {
     "name": "sync-support-secondary-account",
     "owners": [ "treib", "//components/sync/OWNERS" ],
-    "expiry_milestone": 76
+    "expiry_milestone": 80
   },
   {
     "name": "sync-wifi-configurations",