Extend enforce-tls13-downgrade's expiration

We were still waiting on the middleboxes of the world to update so we
can enforce this. However, the reminder is much appreciated since it
looks like the numbers got dramatically better than the last time we
looked, so perhaps this'll be the last time!

Bug: none
Change-Id: Ice4d465cb4c80a90d6631d0151c1b3d653a8431d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1761137
Auto-Submit: David Benjamin <davidben@chromium.org>
Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org>
Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#688501}

chrome/browser/flag-metadata.json

@@ -1955,7 +1955,7 @@
   {
     "name": "enforce-tls13-downgrade",
     "owners": [ "davidben", "svaldez" ],
-    "expiry_milestone": 76
+    "expiry_milestone": 82
   },
   {
     "name": "enterprise-reporting-in-browser",