seboettg/citeproc-php is transferring to the Public Knowledge Project (PKP)

Problem/Motivation

This module currently uses the library seboettg/citeproc-php Per https://github.com/seboettg/citeproc-php/discussions/200#discussioncomme..., this library's maintainership is being transferred to the Public Knowledge Project (PKP), where presumably the new namespace will be pkp/citeproc-php

Once this transfer is complete, we should update our references to use the newly maintained library