Requires warp-lang<13.0

The `pyproject.toml` needs an upper limit for `warp-lang`.

```python
from mujoco_playground import registry

env = registry.load("CartpoleBalance")
```

fails for `warp-lang>=1.13.0` while working for `warp-lang<1.13`

Start in pyproject.toml and inspect the warp-lang dependency constraint. Run the provided CartpoleBalance loading example with the supported and newer warp-lang versions to confirm the incompatibility. Done means the dependency excludes versions at or above 1.13.0 and the example works with an allowed version.

Written by the indexing model from the issue text.