CDDL tests in upstream shelley-spec-ledger-test require a more recent cddl Ruby gem

Summary: `cabal v2-test shelley-spec-ledger` fails if run from the Nix shell of this repository. `shelley-spec-ledger` is defined upstream in the `cardano-ledger-specs` repo.

Some details at input-output-hk/cardano-ledger-specs#1744, which I opened before understanding it well enough to blame the `ouroboros-network` repo.

The `cardano-ledger-specs` Nix files specify `cddl 0.8.10`, whereas the `ouroboros-network` Nix files don't override it, and so inherit the nixpkgs version of `cddl 0.8.9`.

I cannot tell **why** that `.9 -> .10` version rev matters, because I cannot find any data about that project. It seems like the listed owner has turned the GitHub repository private or else deleted it.