Bump version MyST-parser // Sphinx >= 6

### Describe the bug

It is time to upgrade to Sphinx >= 6. 👋🏻 

### Reproduce the bug

Add conflicting project dependencies 💥 

```toml
[project.optional-dependencies]
docs = [
  "myst-nb>=0.17.1",
  "sphinx>=6.1.3", # <-- Banger
  "semver>=3.0.0",
  "furo>=2023.3.27",
  "sphinx-copybutton>=0.5.1",
  "sphinx-design>=0.4.1",
]
```

### List your environment

_No response_