ssi-ucan: outdated libipld dependencies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):

ssi/crates/ucan/Cargo.toml

Line 28 in d031802

libipld = { version = "0.14", default-features = false, features = ["dag-cbor", "dag-json", "derive", "serde-codec"]}

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.