Task-04 uses deprecated `xtimer`

[crasbe]

As the title says.

[benpicco]

Using xtimer as a frontend for ztimer is fine and encouraged as it saves you from monstrosities like

#ifdef MODULE_ZTIMER_MSEC
    ztimer_sleep(ZTIMER_MSEC, x);
#else 
    ztimer_sleep(ZTIMER_USEC, 1000 * x);
#endif