Change fallback for digest algorithm in dnst key2ds from SHA-1 to SHA-256?

[mozzieongit]

Is using SHA-1 as the fallback a good idea? If new algorithms were to get introduced and we forget to update the above list, they would fallback to using SHA-1, right?

Originally posted by @mozzieongit in #2 (comment)

Regarding https://github.com/NLnetLabs/dnst/blob/main/src/commands/key2ds.rs#L224-L235

[tertsdiepraam]

Alternatively, we could remove the fallback altogether, asking the user to pick something