Modernize codebase and add more code quality tools

[indy2kro]

• Added more code quality tools: phpcs, phpstan, rector
• Updated project to be compatible with phpstan level 6
• Added github actions for php 8.2 and 8.3
• Added dependabot github action

[TavoNiievez]

Hi @indy2kro

I agree with the changes you propose in your PR, however there were some that I took the liberty of undoing to continue (See e5de7b0):
For example, adding the rector dependency and its corresponding configuration file.
This specific dependency should not be part of the repository as it is not a linter itself but rather a tool to make refactor, which makes more sense when used locally.
I took the opportunity to remove PHP 8.0 support.
Regarding the typing you added, I had to undo the typing that specifies methods that are defined in an interface.
For congruence, the signature of the method must remain exactly the same in the implementation with respect to the interface.
That's why I lowered the phpstan level to 5 and undid those cases.
I also undid some naming changes that are not so relevant, and some specifications in conditionals that rector suggests but do not make sense in the logic being tested.

If you think I made a mistake in any of these changes please feel free to resubmit them in a new PR, arguing why.

Thank you very much for your contribution.

[indy2kro]

Hello,

Regarding the rector observations, while the tool itself indeed can be used locally, I have learned over time that integrating inside the CI/CD can actually bring benefits - in particular through the PHP version upgrades rector can enforce best practices. However, I understand the reluctance and it is a personal preference in the end. I am happy to contribute even with the other changes which hopefully will bring benefits for others as well.

Thanks for the time and effort for review and merge!