createsend-php library not compatible with php 8.0 or above

Problem/Motivation

Install this module using composer "composer require 'drupal/campaignmonitor:^2.3'"

Steps to reproduce

Navigate to "/admin/config/services/campaignmonitor/lists"
You will get Fatal error: Array and string offset access syntax with curly braces is no longer supported in /app/vendor/campaignmonitor/createsend-php/class/services_json.php on line 156

Proposed resolution

Update the library

Remaining tasks

User interface changes

API changes

Data model changes