Deprecate SHA-1 DS digests in `dnssec-signzone`

Deprecate SHA-1 DS digests in `dnssec-signzone`

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