DEV: Change use of Redis flushall to flushdb.

FLUSHALL removes all keys from all databases. Instead we only want to
remove keys from the current Redis database.

spec/lib/bookmark_reminder_notification_handler_spec.rb

@@ -8,7 +8,7 @@ RSpec.describe BookmarkReminderNotificationHandler do
   fab!(:user) { Fabricate(:user) }
 
   before do
-    Discourse.redis.flushall
+    Discourse.redis.flushdb
   end
 
   describe "#send_notification" do