Ensure compatibility with PHP 8.1

[spawnia]

• Added or updated tests
• Added or updated the README.md
• Detailed changes in the CHANGELOG.md unreleased section

Related Issue/Intent

Allows the library to be installed and used with PHP 8.1. I am currently getting the following error:

Your requirements could not be resolved to an installable set of packages.

  Problem 1
    - bensampo/laravel-enum[v3.2.0, ..., v3.4.2] require laminas/laminas-code ^3.4 -![image](https://user-images.githubusercontent.com/12158000/141855849-53d89947-1ad7-4424-ae50-544cdfc372d4.png)satisfiable by laminas/laminas-code[3.4.0, 3.4.1, 3.5.0, 3.5.1].
    - bensampo/laravel-enum[v1.28.3, ..., v1.38.0] require php ~7.1 -> your php version (8.1.0-dev) does not satisfy that requirement.
    - bensampo/laravel-enum[v2.0.0, ..., v2.2.0] require php ^7.2.5 -> your php version (8.1.0-dev) does not satisfy that requirement.
    - bensampo/laravel-enum[v3.0.0, ..., v3.1.0] require php ^7.3 -> your php version (8.1.0-dev) does not satisfy that requirement.
    - laminas/laminas-code[3.4.0, ..., 3.4.1] require php ^7.1 -> your php version (8.1.0-dev) does not satisfy that requirement.
    - laminas/laminas-code[3.5.0, ..., 3.5.1] require php ^7.3 || ~8.0.0 -> your php version (8.1.0-dev) does not satisfy that requirement.
    - Root composer.json requires bensampo/laravel-enum ^1.28.3 || ^2 || ^3 -> satisfiable by bensampo/laravel-enum[v1.28.3, ..., v1.38.0, v2.0.0, v2.1.0, v2.2.0, v3.0.0, ..., v3.4.2].

#213 was a valid concern, but is no longer a problem.

Changes

Added

• Allow package to be installed with PHP 8.1

Changed

• Allow laminas/laminas-code:^4.0 as a dependency

Breaking changes

None

[spawnia]

Thanks for merging this PR, looking forward to a release 👍

[BenSampo]

Thanks for sorting this, it was on my list but you beat me to it :)

Released under 4.1.0:
https://github.com/BenSampo/laravel-enum/releases/tag/v4.1.0