`load_params` incompatible with `obtax` 0.11.22

Hi team,

It looks like the newest version of `obtax` (0.11.22) causes an issue when calling `gemma.ckpts.load_params`:

```python
Traceback (most recent call last):
***
params = gm.ckpts.load_params(path=ckpt_path)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "***/gemma/gm/ckpts/_checkpoint.py", line 207, in load_params
metadata, path = _get_metadata_and_path(ckpt, path)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "***/gemma/gm/ckpts/_checkpoint.py", line 448, in _get_metadata_and_path
metadata = dict(metadata) # Normalize metadata
^^^^^^^^^^^^^^
TypeError: 'StepMetadata' object is not iterable
```

[This](https://github.com/google/orbax/blob/v0.11.22/checkpoint/orbax/checkpoint/_src/checkpointers/checkpointer.py#L330) is the function in `obtax` that causes the issue.

Reverting back to `obtax` 0.11.21 (or older) resolves the issue.