Extend Assistant app support flag expiration

Extend the expiry milestone of assistant app support feature flag which
is still in use.

Bug: None
Test: Locally build and test
Change-Id: I18e8e08579e80343f2beecaecfaec7394fc1b5b4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1895741Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Commit-Queue: Yue Li <updowndota@chromium.org>
Cr-Commit-Position: refs/heads/master@{#712223}

chrome/browser/flag-metadata.json

@@ -931,7 +931,7 @@
   {
     "name": "enable-assistant-app-support",
     "owners": [ "croissant-eng" ],
-    "expiry_milestone": 78
+    "expiry_milestone": 80
   },
   {
     "name": "enable-assistant-dsp",