Non-DSM Sequoia does not support {rsa2k, nistp*}, reflect that in help message

[zugzwang]

When running a command to generate a local key with an unknown cipher suite, e.g.

sq-dsm key generate --export=priv.key --cipher-suite=xxx

we get the nice help message

error: 'xxx' isn't a valid value for '--cipher-suite <CIPHER-SUITE>'
	[possible values: cv25519, nistp256, nistp384, nistp521, rsa2k, rsa3k, rsa4k]

However, if we pass rsa2k, we get the confusing

$ sq-dsm key generate --export=priv.key --cipher-suite=rsa2k
Error: Unknown cipher suite 'rsa2k'

This is because non-DSM Sequoia does not support RSA2k (or the NIST curves).

The help message should reflect this.