Upgrade sha2 to v0.11

[dmitry-markin]

#448 silences deprecation warnings coming from generic-array v0.14.9. Unfortunately, it is not possible to upgrade generic-array because sha2 < 0.11.0 depends on the version with deprecation warnings.

Once sha2 v0.11 is released, upgrade to it and remove #[allow(deprecated)] from kademlia/types.rs.