Remove support for outdated PHP versions

Nobody has claimed this yet.

The current project supports outdated PHP versions as low as 5.2, which is EOL for more than 11 years now.

Issue #69 showed that is not even possible to build the required test images for these versions anymore. Tests for these versions have been dropped in PR #71.

@sgiehl wrote:
> Feel free to remove PHP 5.6 here as well. We also should update the readme that we require at least PHP 7. Older versions of PHP are meanwhile EOL.

My proposal: Streamline this with the current version of [matomo/matomo and drop support for all versions < 7.2](https://github.com/matomo-org/matomo/blob/4.x-dev/composer.json#L34). They are all EOL.

It is reasonable to keep support for PHP 7.2 as it is part of running distros like Ubuntu 18.04, although it should be recommended everywhere to update to 8.0.

Proposed steps:

* Drop support for all PHP versions < 7.2
* Add this info to the README, recommend 8.0
* Require PHP >= 7.2 in composer
* Drop all code blocks specifically for older versions, e.g. https://github.com/matomo-org/tracker-proxy/blob/master/proxy.php#L124-L134
* Remove all test images with older versions
* Tag a new mayor version (2.0), since this is a breaking change

Start with composer and the README, then inspect the version-specific blocks in proxy.php. Review the test images and the changes from PR #71 to identify older-version coverage that remains. Done means PHP versions below 7.2 are no longer supported, the README recommends PHP 8.0, obsolete code and images are removed, and the project is prepared for a 2.0 release.

Written by the indexing model from the issue text.