Bridge: remove rustix via clap bump (#1109)

Refs: <https://github.com/svix/svix-webhooks/security/dependabot/74>

Formerly we had a runtime dep on a vulnerable version of `rustix`. This
was transitive, and introduced via `clap`. Bumping clap removed this
dependency from our tree.

There is still one vulnerable version introduced via
`opentelemetry-otlp`, but since it's a build-dep it's less of a concern
(see the advisory for the rationale).

```
$ cargo tree -p [email protected] -i
rustix v0.38.8
└── tempfile v3.7.1
    └── prost-build v0.11.9
        └── tonic-build v0.8.4
            [build-dependencies]
            └── opentelemetry-proto v0.1.0
                └── opentelemetry-otlp v0.11.0
                    └── svix-bridge v1.13.0 (/home/onelson/Projects/svix-webhooks/bridge/svix-bridge)
```

If we can update the various otel-related deps, we might be able to bump
`rustix` or remove it entirely, but that's a bigger lift.