Compatibility for some parts of the code with PHP 8.1

[victorjkhalaf]

I found some areas where PHP deprecation notices were being thrown due to the new stricter "Passing null to non-nullable internal function parameters is deprecated." change (https://www.php.net/releases/8.1/en.php) while using the package. There were also issues with FluidContext requiring the \Iterator interface's enforced return types be declared in the overridden methods inside the class so I added the return types. This does not completely port the codebase to where it ought to get to, but I didn't want to also have a fork that remained separate for things that should be included in maintaining this package.

I also bumped minimum version requirement to PHP 7.4 to ensure the compatibility with return types is available.