boards: nrf54h20dk: Merge iron variants into the base variants

This replaces the legacy SDFW compatible board configuration with the
IronSide SE compatible one, thus removing support for running samples
and tests on nRF54H20 devices with the old firmware.

All applications are expected to work on `nrf54h20dk/nrf54h20/cpuapp`
out of the box. For other board targets, all applications are expected
to boot, but may require additional peripheral configuration in UICR.
Build system support for the new UICR format is to be added separately.

Co-authored-by: Jonathan Nilsen <jonathan.nilsen@nordicsemi.no>
Signed-off-by: Jonathan Nilsen <jonathan.nilsen@nordicsemi.no>
Signed-off-by: Grzegorz Swiderski <grzegorz.swiderski@nordicsemi.no>