PHP 8 upgrade

[RomainOdeval]

• Force PHP version with composer.json
• Remove all non-complient PHP8 / deprecated code
• Change PHPDoc into PHP8 annotations
• Create a GitHub release ?

[RomainOdeval]

First part (Force PHP version with composer.json + Remove all non-complient PHP8 / deprecated code) resolved by #10
A few PHP8 have been added too but PHPDoc removal will be done with another PR

[WebCrew]

• Create a GitHub release ?

I think thats a good idea to keep things orderly and clear.

Thanks for your work Romain.