Update darken-websites-checkbox-in-themes-setting flag expiration

Extend to M92

Bug: NONE
Change-Id: I0e2ee3f83ebc6e0e553010ca997ee1493b7dba59
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2351717
Auto-Submit: Lijin Shen <lazzzis@google.com>
Reviewed-by: Theresa  <twellington@chromium.org>
Commit-Queue: Lijin Shen <lazzzis@google.com>
Cr-Commit-Position: refs/heads/master@{#797449}

chrome/browser/flag-metadata.json

@@ -749,7 +749,7 @@
   {
     "name": "darken-websites-checkbox-in-themes-setting",
     "owners": [ "lazzzis@google.com", "twellington" ],
-    "expiry_milestone": 83
+    "expiry_milestone": 92
   },
   {
     "name": "dcheck-is-fatal",