DEV: use Discourse.cache over Rails.cache

Discourse.cache is a more consistent method to use and offers clean fallback
if you are skipping redis

This is part of a larger change that both optimizes Discoruse.cache and omits
use of setex on $redis in favor of consistently using discourse cache

Bench does reveal that use of Rails.cache and Discourse.cache is 1.25x slower
than redis.setex / get so a re-implementation will follow prior to porting

spec/components/site_setting_extension_spec.rb

@@ -800,7 +800,7 @@ describe SiteSettingExtension do
 
     it 'expires the cache' do
       settings.default_locale = 'zh_CN'
-      expect(Rails.cache.exist?(SiteSettingExtension.client_settings_cache_key)).to be_falsey
+      expect(Discourse.cache.exist?(SiteSettingExtension.client_settings_cache_key)).to be_falsey
     end
 
     it 'refreshes the client' do