Key4::with_secret and Key6::with_secret can make unsupported keys

sequoia_openpgp::packet::key::Key6::with_secret can be used to construct DSA and Ed25519Legacy Keys, which are unsupported for v6 signatures.

sequoia_openpgp::packet::key::Key4::with_secret can be used to construct Ed25519 (non-legacy) Keys, which are unsupported for v4 signatures.

Since Key4::with_secret and Key6::with_secret return Results, it would make sense for them to fail on issues like this.

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