Fix PHPUnit 10 compatibility

[rgson]

• Adds type hints to tests to deal with errors like:

.PHP Fatal error: Declaration of Carbon\Doctrine\CarbonTypeConverter::getSQLDeclaration(array $fieldDeclaration, Doctrine\DBAL\Platforms\AbstractPlatform $platform) must be compatible with Doctrine\DBAL\Types\DateTimeType::getSQLDeclaration(array $column, Doctrine\DBAL\Platforms\AbstractPlatform $platform): string in src/Carbon/Doctrine/CarbonTypeConverter.php on line 37

• Adds a workaround for a type mismatch in a trait shared between two classes in need of different return type signatures.

.PHP Fatal error: Declaration of Carbon\Doctrine\CarbonTypeConverter::convertToPHPValue(mixed $value, Doctrine\DBAL\Platforms\AbstractPlatform $platform): ?DateTimeInterface must be compatible with Doctrine\DBAL\Types\VarDateTimeImmutableType::convertToPHPValue(mixed $value, Doctrine\DBAL\Platforms\AbstractPlatform $platform): ?DateTimeImmutable in src/Carbon/Doctrine/CarbonTypeConverter.php on line 69

• Replaces the removed assertObjectHasAttribute method.

[kylekatarnls]

This would drop completely the PHP 7 compatibility. But 3.x version is compatible with PHPUnit 10.