Increasing the minimum PHP version

[kudashevs]

Hello @Donatello-za ,

I tested the package on PHP 7.1, 7.2, 7.3 and higher (it works). I wasn't able to test it on 7.0 and lower.
So, I would like to suggest increasing the minimum supported version to 7.4. Ideally to 8.0 or 8.1.
This will give the possibility to use some new PHP features and make code base more strict and type oriented.

If you're up to, I can help in this process. Let me, please know, about your decision.