Support PHP 7.4+

[gaglioffo]

Hi,
you consider migrating and rewriting some part the code to be compatible also with PHP 7.4 and 8 right now

https://www.php.net/supported-versions.php

An example to use curly brace syntax for accessing array elements and string offsets it's deprecate on 7.4 into MailWizzApi\Json.php and MailWizzApi\Base.php
https://wiki.php.net/rfc/deprecate_curly_braces_array_access

Thanks a lot

[twisted1919]

The next generation API client is at https://github.com/ems-api/php-client and will be used for MailWizz 2.0.
For this particular client, we're open to pull requests to bring it to date.