Remove support for PHP 5.6

[nathansalter]

Based on this table PHP 5.6 has been out of active support for a long time. Looking at our current supported versions we should be a bit more explicit in this for the composer.json file. Looking at an analysis of the estimated version usage My suggested supported versions would be as follows:

• 7.4
• 8.0
• 8.1
• 8.2
• 8.3

Basically having a cut-off of 5% usage stats to remove that version. I do appreciate that some of our customers in particular might still be on PHP 7.2 but looking at our Packagist stats I don't think that's actually the case. It actually seems like the only system using this library on 5.6 is our auto-tests.

The features we'd be able to use by moving to 7.4 are:

• Scalar type hints (function foo(string $bar, int $rawr)
• Scalar property types (private string $foo)
• Variadics [$a, $b] = ...$list and function foo(string ...$ids)
• Libsodium (random_bytes() as a timing-safe random number generator)

Which I think would make the PHP library at least look a lot more like our other libraries.