Update legacy-tls-enforced flag expiry

Bumps the flag for the Legacy TLS interstitial (on Desktop) to expire in
M-92, which is the release after we plan to remove support in May 2021.

Change-Id: I7b3e00c1777b2a9c8a50d4f4c223459729b708b1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2508634
Auto-Submit: Chris Thompson <cthomp@chromium.org>
Commit-Queue: Carlos IL <carlosil@chromium.org>
Reviewed-by: Carlos IL <carlosil@chromium.org>
Cr-Commit-Position: refs/heads/master@{#822932}

chrome/browser/flag-metadata.json

@@ -3014,7 +3014,7 @@
   {
     "name": "legacy-tls-enforced",
     "owners": [ "cthomp" ],
-    "expiry_milestone": 87
+    "expiry_milestone": 92
   },
   {
     "name": "legacy-tls-interstitial",