This changeset enables compatibility with PHP 8.4+ while retaining compatibility as far back as PHP 7.3.

The primary concern is the nesbot/carbon version dependency, which needs to be v3.x for PHP 8.4+.

To achieve this, the unused PHPStan dependencies and config were removed, and the PHP_CS dependencies and config updated, with minor fixups to formatting in some code.

We can add PHPStan back into the mix in the future.

Testing:

• Switch to PHP 7.3
• make clean-all && make test
• Switch to PHP 7.4
• make clean-all && make test
• ... and so on all the way up to PHP 8.5

Check that PHP 7.x uses nesbot/carbon 2.x while PHP 8.x uses nesbot/carbon 3.x.

Use this branch in a real project to test that it works without error and enables usage of Carbon v3 when using PHP 8.1+ (I'll raise a follow-on PR on another project for this).