[php84] Fix some deprecations and lower requirements

[Jubeki]

I send the PR to the php84 branch, or should it go directly to 11.x branch?

Is there a StyleCI Rule to enforce explicit nullable types?
A Quick Look / search on https://docs.styleci.io/fixers resulted in nothing.

PHP-CS-Fixer has a rule for this (which is also enforced in Laravel Pint)
https://cs.symfony.com/doc/rules/function_notation/nullable_type_declaration_for_default_null_value.html

Lower Requirements for PHP 8.4:

• guzzle/promises should be ^2.0.3
• mockery/mockery should be ^1.6.10
• symfony/* should be ^7.1.3 or ^7.0.10
• sebastian/cli-parser should be ^2.0.1 for PHPUnit 10 and ^3.0.1 for PHPunit 11
• nesbot/carbon should be ^3.4.0
• sebastian/exporter should be ^6.0.1

I also adjusted the PHPunit requirements in Github Workflows accordingly for nullable types.

[Jubeki]

Starting Point:
PHPUnit 11 - lowest: Errors: 10, Failures: 8, Deprecations: 735

After this PR:
PHPUnit 11 - stable: Errors: 9, Failures: 8, Deprecations: 68

(Not completely a fair comparison due to using lowest vs stable, but that is okay for me)

[driesvints]

Very nice. Thanks @Jubeki

[driesvints]

Btw, we can also bump these dependencies inside our own composer.json

[Jubeki]

Yeah I know, I just wanted to find out the lower bounds for now, without needing to update all composer.json files.