Set context types of MailRoom's postback requests to text/plain in the upstream gem

A follow up of https://gitlab.com/gitlab-org/gitlab/-/issues/362068.

In the above issue, the content type of MailRoom postback requests are set to text/plain in all GitLab distributions. It makes sense to update the default value in the gem as well: https://github.com/tpitale/mail_room/blob/14e5bccac93493ad3e84d56140ee172974365dcf/lib/mail_room/delivery/postback.rb#L98. After the patch is accepted, we will need to back-port the patch back to GitLab's fork, and bump versions accordingly.