Update polling serializers for merge request widget

There are 2 API serializers used in the merge request widget which poll for information about the MR. These currently check the project's settings for the squash option. We need to update these to fetch the value from the merge request instead

https://gitlab.com/gitlab-org/gitlab/-/blob/master/app/serializers/merge_request_poll_widget_entity.rb

https://gitlab.com/gitlab-org/gitlab/-/blob/master/app/serializers/merge_request_poll_cached_widget_entity.rb

  expose :squash_enabled_by_default do |merge_request|
    presenter(merge_request).project.squash_enabled_by_default?
  end

  expose :squash_readonly do |merge_request|
    presenter(merge_request).project.squash_readonly?
  end

  expose :squash_on_merge do |merge_request|
    presenter(merge_request).squash_on_merge?
  end