fix DateTime -> DateTimeImmutable

[Gemorroj]

psr clock is immutable: https://github.com/php-fig/clock/blob/master/src/ClockInterface.php#L12

Error thrown while running command "shapecode:cron:run". Message: "Could not convert PHP value of type Symfony\Component\Clock\DatePoint to type Doctrine\DBAL\Types\DateTimeType. Expected one of the following types: null, DateTime." {"exception":"[object] (Doctrine\\DBAL\\Types\\Exception\\InvalidType(code: 0): Could not convert PHP value of type Symfony\\Component\\Clock\\DatePoint to type Doctrine\\DBAL\\Types\\DateTimeType. Expected one of the following types: null, DateTime. at /var/www/xxx/vendor/doctrine/dbal/src/Types/Exception/InvalidType.php:51)","command":"shapecode:cron:run","message":"Could not convert PHP value of type Symfony\\Component\\Clock\\DatePoint to type Doctrine\\DBAL\\Types\\DateTimeType. Expected one of the following types: null, DateTime."} []