bug(COMPONENT): docs components/tree/examples Tree with dynamic data esample FlatTreeControl deprecated

### Is this a regression?

- [ ] Yes, this behavior used to work in the previous version

### The previous version in which this bug was not present was

_No response_

### Description

docs components/tree/examples Tree with dynamic data esample FlatTreeControl deprecated

### Reproduction

StackBlitz link:
Steps to reproduce:
1.
2.

### Expected Behavior

New Example Method

### Actual Behavior

FlatTreeControl is deprecated

Flat tree control. Able to expand/collapse a subtree recursively for flattened tree.

@deprecated
Use one of levelAccessor or childrenAccessor instead. To be removed in a future version.

@breaking-change — 21.0.0

### Environment

- Angular: 18
- CDK/Material:18
- Browser(s): edge
- Operating System (e.g. Windows, macOS, Ubuntu): windows

Beginne in docs components/tree/examples und untersuche das Beispiel „Tree with dynamic data“ mit FlatTreeControl. Ersetze den veralteten Ansatz durch die dokumentierte levelAccessor- oder childrenAccessor-Methode und überprüfe anschließend, dass das Beispiel weiterhin die dynamische Erweiterung und das Zuklappen eines flachen Baums demonstriert.

Vom Indexierungsmodell aus dem Issue-Text verfasst.