This affects two cases:
* When writing a `dsset` file for this zone, to be used by its
parent, only write a SHA-256 DS record.
* When reading a `keyset` file for a child, to generate DS records
to include in this zone, generate SHA-256 DS records only.
This change does not affect digests used in CDS records.
This is for conformance with the DS/CDS algorithm requirements in
https://tools.ietf.org/html/draft-ietf-dnsop-algorithm-update
d8f2eb249a