[Feature Request]: Add a function to call DELETE on documents/olderthan

### Current Behavior

Coveo has a feature to delete documents that are older than a given date. When doing a full refresh, that endpoint can be called to delete documents that don't exist anymore.

The endpoint documentation: https://docs.coveo.com/en/131/index-content/deleting-old-items-in-a-push-source

The current SDK implementation does not expose this method.

### Expected Behavior

The SDK should expose this method.

### Steps To Reproduce

1. Install the SDK
2. Look for methods available on a Source
3. The method is not there

### Environment

```Markdown
Python Version -> Latest
OS Version -> Latest macOS
...
```

### Debug logs

_No response_

### Anything else?

Here is a bad piece of code I ended up using:
```python
url = (
f"https://api.cloud.coveo.com/push/v1/organizations/{self.organization_id}/sources/{self.source_id}/"
f"documents/olderthan"
)
response = self.source.client.session.delete(
url,
params={"orderingId": self.clear_items_older_than},
headers={"Authorization": f"Bearer {self.coveo_api_key}"},
)

if response.status_code == 202:
logger.info("Old documents deleted successfully.")
else:
logger.error(f"Failed to delete old documents: {response.json()}")
```

Start at the SDK's Source entry point and inspect how existing source methods build requests and expose response handling. Add coverage for the documents/olderthan DELETE endpoint, including its orderingId parameter and 202 response, and confirm the method is available on Source.

Written by the indexing model from the issue text.