Fix PeriodAbstract constructor when DateTimeImmutable instances are passed

[mpdude]

When DateTimeImmutable instances are used, the $this->end->add(...) call will not modify $this->end. It does not matter that a clone is used.

[mpdude]

Probably the issue is much bigger, potentially all places that clone ...$someDate before using it made the false assumption that this will work for DateTimeImmutable instances.

[peter279k]

According to the official PHP site, the DateTime::createFromInterface method is only available since the PHP 8.0 version.

And this PR will be broken in PHP 7.x versions.

[mpdude]

Any suggestion how this could be worked around?

[peter279k]

To resolve this issue, I've two suggestions:

1. Using the method_exists to check whether the DateTime::createFromInterface is available. If not, using the original method call.
2. Dropping PHP 7.x versions support for next released version.

Do you have any other recommendations about this?