Deprecate prefix signatures

From @vanitasvitae elsewhere:

I'd be in favor of deprecating [prefix signatures]. It makes no sense to have two mechanisms that do the same thing when one of them is even superior without much overhead.

It seems that prefix signatures are rare in practice: https://sequoia-pgp.gitlab.io/openpgp-interoperability-test-suite/results.html#Sign_Encrypt-Decrypt_Verify_roundtrip_with_key__Alice_

We could say:

• v4 prefix signatures SHOULD NOT be generated.
• v6 prefix signatures MUST NOT be generated.
• v4 prefix signatures MUST NOT be mixed with OPS signatures (of any version).
• v4 prefix signatures MAY be converted to OPS form by a receiving implementation (say, if it wants to add another signature to the same message).