ssi-ucan: outdated `libipld` dependencies

[lucab]

ssi-ucan currently depends on a stale version of libipld (latest version at this time is 0.16.0): https://github.com/spruceid/ssi/blob/d0318022b31181e08656e46e49b55704fe42c54d/crates/ucan/Cargo.toml#L28

This stale version also brings in a chain of old dependencies, which would be good to avoid.

However, this bump may be a breaking change, as dependency types seem to appear in the public interfaces, like StoreParams in https://docs.rs/ssi-ucan/0.2.1/ssi_ucan/struct.Ucan.html#method.to_block.

[vmx]

As libipld is deprecated, I'm currently looking into repositories still using it. I then try to help with PRs.

The concept of a Block doesn't exist in ipld-core (the replacement for libipld) anymore. It's part of the public interface. Who is the user of that interface?