Explain version requirements for `composer-normalize`

[oliverklee]

#1390 (comment)

composer-normalize seems to throw a false error: 'Your Composer dependencies require a PHP version ">= 7.4.0"'.

The message is misleading, since we install composer-normalize via PHIVE. composer-normalize does not follow semver: version 2.21.0 dropped support for PHP 7.3, which is why the error occurs.

Adjusting the require-dev section of composer.json to require PHP >= 7.4 would not be viable, since that would prevent running the unit tests on 7.3. But an explanatory note in the contributing guidlines would probably not go amiss...

[JakeQZ]

We probably need to add in the contributing guidelines that PHP >= 7.4 is needed for some of the dev tools (PHPStan as well as Composer-Normalize, possibly others), though the unit tests can be run on any supported PHP version (and will be run on all in the CI build), and try to keep this up to date. I think the same will apply to PHP-CSS-Parser.