\Drupal\Component\Datetime\DateTimePlus should pass correct parameter types to checkdate()

Problem/Motivation

In \Drupal\Component\Datetime\DateTimePlus::checkArray() we do

      if (@checkdate($array['month'], $array['day'], $array['year'])) {
        $valid_date = TRUE;
      }

The $array['month'], $array['day'], $array['year'] values are not guaranteed to be integers and they should be. In PHP8 this causes test failures.

This error is produced when running Drupal\Tests\datetime\Functional\DateTimeFieldTest::testDatelistWidget see https://dispatcher.drupalci.org/job/drupal_patches/51890/testReport/juni...

Proposed resolution

Casting each value to an integer works but is it the best fix?

This issue should also be backported to 8.9.x so that Drupal 8 can be PHP 8.0 compatible.

Remaining tasks

User interface changes

N/a

API changes

N/a

Data model changes

N/a

Release notes snippet

N/a