[Bug] Error when updating fitted model using ``stan_init``

**Bug description**

When trying to update my model fit with new data using ``updated fitted model`` section here (https://facebook.github.io/prophet/docs/additional_topics.html), I get the following error

`Traceback (most recent call last):
File "", line 1, in
File "/virtualenvs/smartshift-load-forecasting-9TtSrW0h-py3.9/lib/python3.9/site-packages/prophet/forecaster.py", line 1171, in fit
self.params = self.stan_backend.fit(stan_init, dat, **kwargs)
File "/virtualenvs/smartshift-load-forecasting-9TtSrW0h-py3.9/lib/python3.9/site-packages/prophet/models.py", line 90, in fit
kwargs['inits'] = self.prepare_data(kwargs['init'], stan_data)[0]
File "/virtualenvs/smartshift-load-forecasting-9TtSrW0h-py3.9/lib/python3.9/site-packages/prophet/models.py", line 164, in prepare_data
'y': data['y'].tolist(),
AttributeError: 'list' object has no attribute 'tolist'`

Note: Potentially a similar issue mentioned here: https://issuemode.com/issues/facebook/prophet/104552814.

**Reproducible example**

```
def stan_init(m):
res = {}
for pname in ['k', 'm', 'sigma_obs']:
res[pname] = m.params[pname][0][0]
for pname in ['delta', 'beta']:
res[pname] = m.params[pname][0]
return res

def update_model_state(input_data, fitted_model, intervals_per_day=None):
"""Update model state of fitted model without updating parameters"""

old_data = fitted_model.history[['ds', 'y']]
updated_data = pd.concat([old_data, input_data], axis=0).reset_index(drop=True)

updated_model = Prophet(
seasonality_mode='multiplicative',
yearly_seasonality=False,
weekly_seasonality=False,
daily_seasonality=True,
).fit(
updated_data,
init=AutoArimaProphet.stan_init(fitted_model) # warm fit
)
return updated_model

if __name__ == '__main__':

read in fb_input_data ...

estimator = Prophet(
seasonality_mode='multiplicative',
yearly_seasonality=False,
weekly_seasonality=True,
daily_seasonality=True,
)
fitted_model = estimator.fit(fb_train_data)

fitted_model = update_model_state(
input_data=fb_input_data,
fitted_model=fitted_model
)
```

Unable to provide data since sensitive, but here is a santiized excert.

ds y
0 2022-03-01 00:00:00 7.677567
1 2022-03-01 00:30:00 7.840946
2 2022-03-01 01:00:00 7.888173
3 2022-03-01 01:30:00 7.870759
4 2022-03-01 02:00:00 7.714092