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

Start in docs components/tree/examples and inspect the Tree with dynamic data example using FlatTreeControl. Replace the deprecated approach with the documented levelAccessor or childrenAccessor method, then verify that the example still demonstrates dynamic flat-tree expansion and collapse.

Written by the indexing model from the issue text.