Use Postfinance API v3

Please can this be merged. V2 Api generates a shit-ton of logs with php 8 due to deprecations.

@berdir Any chance this can be commited soon ? There is so much error on PHP 8.1 without it, that it make this module almost unusable. Thanks.

For maximum compatibility with existing sites, let's declare a version constraint which allows both major versions of the SDK.

Automatically closed - issue fixed for 2 weeks with no activity.

znerol → closed merge request !6