Extend flag "use-multilogin-endpoint" until M88.

This flag is used to enable using the Multilogin endpoint on desktop
platforms.

Bug: None

Change-Id: If7add4d0adb85904fe61c782c65b480b7cc606dc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2362899Reviewed-by: David Roger <droger@chromium.org>
Commit-Queue: Mihai Sardarescu <msarda@chromium.org>
Cr-Commit-Position: refs/heads/master@{#799580}

chrome/browser/flag-metadata.json

@@ -4430,7 +4430,7 @@
   {
     "name": "use-multilogin-endpoint",
     "owners": [ "droger", "msarda" ],
-    "expiry_milestone": 85
+    "expiry_milestone": 88
   },
   {
     "name": "use-preferred-interval-for-video",