Increase feature flag cache TTL to one hour

Flipper already takes care of flushing cache entries when
enabling/disabling features so it should be safe to increase the TTL.
This in turn should drastically reduce the number of Flipper queries
executed.

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/40854

changelogs/unreleased/flipper-caching.yml

+---
+title: Increase feature flag cache TTL to one hour
+merge_request:
+author:
+type: performance

config/initializers/flipper.rb

@@ -8,7 +8,7 @@ Flipper.configure do |config|
     cached_adapter = Flipper::Adapters::ActiveSupportCacheStore.new(
       adapter,
       Rails.cache,
-      expires_in: 10.seconds)
+      expires_in: 1.hour)
 
     Flipper.new(cached_adapter)
   end