8.4 deprecates implicitly nullable parameters

This library however supports PHP 7.2 which does not support explicit nullable types (i.e. ?string) and was end-of-life over 4 years ago.

This PR:

• Bumps the minimum PHP version to 8.0 (which is also end-of-life, maybe a good move to go to 8.1?)
• Introduces rector as a dev dependency to facilitate these types of refactoring.
• Adds switches from implicitly nullable to explicitly nullable parameters.

cc @roblourens @TysonAndre what do you think?