Remove support for outdated PHP versions

[pixelbrackets]

The current project supports outdated PHP versions as low as 5.2, which is EOL for more than 11 years now.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

[sgiehl]

@pixelbrackets I would fully agree with your proposed steps.
@tsteur @mattab Do you see any problems in dropping support for older PHP versions here?

[tsteur]

Since it's a small project and people might include it in all kind of setups it would be great to ideally still support PHP 7.0 and newer from my perspective and it be fine to drop support for PHP 5.X as long as we release a new major version then.

Would it cause any issues to also support PHP 7.0 and 7.1 re the tests?