Removes patches that are already in Rails 6:
Gitlab::ContentDisposition patch
ActiveRecord::QueryCache patch
ActiveJob::Arguments patch
Gitlab::ActionViewOutput patch

Updates Activerecord::Locking::Optimistic patch to follow changes in
https://github.com/rails/rails/blob/v6.0.1/activerecord/lib/active_record/locking/optimistic.rb

Fix missing timezone for datetime_with_timezone by adding this custom
type to ActiveRecord's time_zone_aware_attributes