PHP versions too generous about phpstan-cakephp2

[ariddlestone]

I'm going to set the required php version to 7.4.

phpstan is broken for php < 7.4, with no easy way to fix it. The steps required would be:

• better-reflection would need to release a new php 7.2 compatible version, despite now only supporting php 8.x
• phpstan would need to release a new php 7.2 compatible bundle with the new php 7.2 better-reflection, but that would mean steps backwards in features etc., so would need to be kept separate from the main releases

As PHP 7.2 & 7.3 have not been supported in a while, the work above is going to be considered disproportionate compared to our little library!

This is a breaking change, as despite all the issues listed above, installing the package itself in PHP 7.2 succeeds.