Bump dependencies in docs environment

**Description**
* `myst-parser` is stuck on 3.x but 4.x is released
* `sphinx` is pinned to 6.1.3 which is also out of date

https://github.com/openforcefield/openff-interchange/blob/c68c147d128204b33527890277b312ef2e7e67de/devtools/conda-envs/docs_env.yaml