[make:entity] Use stricter DateTime type instead of DateTimeInterface for mutable datetime fields

[rosier]

The current DateTimeInterface type hint suggest that you can put a DateTimeImmutable in a mutable datetime field, but that is no longer the case.

It results in the following error:

Could not convert PHP value of type DateTimeImmutable to type Doctrine\DBAL\Types\DateTimeType. 
Expected one of the following types: null, DateTime.

Follows:

• Narrow responsibilities in datetime related types doctrine/dbal#6017
• Stricter DateTime types doctrine/dbal#6161

[kbond]

Makes sense!

[kbond]

Thank you @rosier.