boot: zephyr: boards: update name of nrf54h20dk

The nrf54h20dk no longer has the variant "iron". "iron" has become
the default so the name of the target is now just
nrf54h20dk_nrf54h20_cpuapp.

Signed-off-by: Bjarki Arge Andreasen <bjarki.andreasen@nordicsemi.no>