Bump signature v2.1.0 to v2.2.0

[gilescope]

non-functional signature 2.1 -> 2.2 change detailed here: https://github.com/RustCrypto/traits/blob/master/signature/CHANGELOG.md

and the ecdsa version bump is just loosening the requirement on signature:
https://github.com/RustCrypto/signatures/blob/master/ecdsa/CHANGELOG.md

(We could just bump ecdsa crate so it can resolve and work with signature 2.2 without being forced to, but the signature 2.2 change looks like a low risk change we should pick up.)

Not sure if it's PRdoc-worthy?

[bkchr]

Pure Cargo.lock changes are not really required, especially for these non changes.