[13.x] Improve collection return types for methods that mutate

[axlon]

PHPStan 2.2.14 ships a bugfix that fixes that PHPStan could not understand (T of array-key)|int. Since this is now fixed some Collection methods that mutate the collection directly can have their types improved.

Before:

collect(['foo' => 'bar'])->push('baz'); // Collection<string, string>

After:

collect(['foo' => 'bar'])->push('baz'); // Collection<int|string, string>