Fix mypy errors with latest canonicaljson (#13905)

* Lockfile: update canonicaljson 1.6.0 -> 1.6.3

* Fix mypy errors with latest canonicaljson

The change to `_encode_json_bytes` definition wasn't sufficient:

```
synapse/http/server.py:751: error: Incompatible types in assignment (expression has type "Callable[[Arg(object, 'json_object')], bytes]", variable has type "Callable[[Arg(object, 'data')], bytes]")  [assignment]
```

Which I think is mypy warning us that the two functions accept different
sets of kwargs. Fair enough!

* Changelog