Form date field doesn't support CarbonImmutable

[mxaGianluca]

When passing an immutable Carbon object into the Form::date field, it will try and validate the input by checking if it's an instance of DateTime which doesn't pass.

html/src/FormBuilder.php

Line 434 in ae15b9c

if ($value instanceof DateTime) {

This could be fixed by checking if the value implements DateTimeInterface, then it would support both.