zipl: Switch from genprotimg to pvimg for SE boot image generation

zipl: Switch from genprotimg to pvimg for SE boot image generation

el9, el10 and rhel-9.6 now ship s390utils-base >= 2.36.0, which
replaces the obsolete genprotimg tool with pvimg. Update the Secure
Execution boot image generation to use 'pvimg create' with long-form
flags (--kernel, --ramdisk, --parmfile, --output).

The pvimg tool natively supports the --overwrite option, eliminating
the need to manually remove the existing sdboot image before
regeneration.