PHP 8.1 support: fix deprecation notices

[judgej]

Examples:

Return type of Consilience\Starling\Payments\AbstractResponse::jsonSerialize() should either be compatible with JsonSerializable::jsonSerialize(): mixed, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice in /.../starling-payments-objects/src/HydratableTrait.php on line 220

Return type of Consilience\Starling\Payments\AbstractServerRequest::jsonSerialize() should either be compatible with JsonSerializable::jsonSerialize(): mixed, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice in /.../starling-payments-objects/src/HydratableTrait.php on line 220

A bit of static analysis may throw up a few more.

[judgej]

Upgrade PHP money 3.3 to 4.04 for full PHP 8.1 compatibility.

Requires the BC Math extension.