[bfcache] Update the flag expiration milestone to M95

Back-forward cache is in the process of being shipped and
under active further development, so this flag is going to be useful
for the foreseeable future.

R=fergal@chromium.org

Change-Id: I5c72a74dd7f7f7ed015ef4dd2942eb75718f9733
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2574983
Commit-Queue: Alexander Timin <altimin@chromium.org>
Commit-Queue: Fergal Daly <fergal@chromium.org>
Auto-Submit: Alexander Timin <altimin@chromium.org>
Reviewed-by: Fergal Daly <fergal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#834559}

chrome/browser/flag-metadata.json

@@ -400,7 +400,7 @@
   {
     "name": "back-forward-cache",
     "owners": [ "bfcache-dev@chromium.org" ],
-    "expiry_milestone": 88
+    "expiry_milestone": 95
   },
   {
     "name": "background-task-component-update",