we no longer need v3.15.8 of protobuf. we should use v3.21.12 everywhere.
additionally, we don't seem to use blob.nix anywhere. it's been broken for months since 3.15.8 was removed from the nix dev environment in D8843, so i think it's safe to remove entirely.
note that protobuf changed their branch naming so v21.12 is actually semver 3.21.12