Databricks SDK: ClustersAPI.create missing data_security_mode

## The symptom

Calling `ClustersAPI.create` with `data_security_mode` failed: the attribute
was missing from the SDK's generated model. Cluster policies requiring the
field broke SDK-based creation.

## Why

The field had been added to the OpenAPI spec but the installed SDK was
generated from an older spec. The maintainer confirmed the field is present in
the latest SDK release.

## The fix

```
pip install -U databricks-sdk
```

Then pass the mode directly:

```python
w.clusters.create(..., data_security_mode=DataSecurityMode.SINGLE_USER)
```

Use whatever mode your policy requires. If your org pins an old SDK version,
that pin is the bug: the field only exists in the newer generated code.

## The general lesson

When a documented API field is "missing" from an SDK, check the SDK version
before anything else. Generated SDKs lag the spec, and the fix is an upgrade,
not a workaround.