php-cmis is abandonned

Problem/Motivation

This module depends on the php-cmis library which is abandoned: https://packagist.org/packages/dkd/php-cmis

Steps to reproduce

Run composer require drupal/cmis:^2.0@alpha dkd/php-cmis:dev-master.
You get this warning:

Package dkd/php-cmis is abandoned, you should avoid using it. No replacement was suggested.

Proposed resolution

The module should switch to a maintained CMIS library.
If that is not possible, the module should probably be marked as deprecated.