age: Remove `Identity::unwrap_stanza`

[str4d]

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Per https://github.com/str4d/rage/pull/507#discussion_r1703367459, I'm going to remove Identity::unwrap_stanza, and make Identity::unwrap_stanzas a required method. The latter will also be documented as always receiving all stanzas from the same file (rather than the current documentation that this is assumed).

[str4d]

Hmm, after deciding to do this, I took another look at @FiloSottile's age and realised that although its Identity only exposes multi-stanza unwrapping, every single impl uses an internal multiUnwrap helper that does exactly what the default impl of Identity::unwrap_stanzas does: https://github.com/str4d/rage/blob/a510e76bf8b5ba1c31d873c38fd1b8056091538f/age/src/lib.rs#L216-L218

The Rust impl is significantly cleaner because Rust has Option and can return Option<Result<_, _>>, whereas Go matches on the returned err looking for a sentinel ErrIncorrectIdentity that serves the same role as None in our API.

So, I'm now back on the fence about whether to remove Identity::unwrap_stanza. It's currently the case that Decryptor only ever calls Identity::unwrap_stanzas, so we could just document on Identity that its canonical entry point is Identity::unwrap_stanzas, and then identity impls that want to add file-level stanza checks can do so by overriding the default impl.