Switch to Mollie API v3 (on a new 2.x branch?)

Problem/Motivation

The is a new V3 available for Mollie API client for PHP, see https://github.com/mollie/mollie-api-php/

Proposed resolution

Switch to Mollie API v3 (on a new 2.x branch?)

Remaining tasks

1. Write a merge request
2. Review
3. Commit

User interface changes

None

API changes

To be determined.

Data model changes

To be determined.