Remove deprecated warning on strftime on due date extension (#67)

[voegii]

Solves #67

[Lyannic]

Thank you very much for you contribution.

I am not quite sure whether using the userdate is a good idea here.
The docs from the Moodle Time API says

userdate - Gets formatted string that represents a date in user time (note that the format required by this function is the [strftime()](https://www.php.net/manual/en/function.strftime.php) format, not the more common format used by date())

From what I understand, this means that the date is displayed in the user's time zone. We normally don't use this at other places.

This is for example how we display the normal due date for a MUMIE-Task in mod/mumie/view.php

        $duedateinfo .= html_writer::tag(
            'p',
            strftime(
                get_string('strftimedaydatetime', 'langconfig'),
                $mumietask->duedate
            ),
            array('style' => 'font-weight: bold; margin-top:10px;')
        );

If using userdate means that the due date is converted into the user's time zone, I think that it is confusing to potentially display dates from different time zones, since we don't use the userdate at other locations.