[@mantine/dates] DateTimePicker: prevent converting value to DateProvider twice

[mrbnclt]

DateTimePicker uses useUncontrolledDates hook which is already using the context from the DateProvider to convert the default value into the DateProviders timezone.

In the DateTimePicker we shouldn't do another .tz() that will convert the date & time into the DateProvider's timezone, the result will be incorrect if that's the implementation.

Fix was to pass true to the .tz()'s second parameter (keepLocalTime), which doesn't convert the value again to the DateProvider's timezone.

Before

After

Closes #7388

[rtivital]

Please fix errors in tests