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

docs components/tree/examples から始め、FlatTreeControl を使用している Tree with dynamic data の例を調べます。非推奨のアプローチを、ドキュメントに記載されている levelAccessor または childrenAccessor メソッドに置き換え、その後も例が動的なフラットツリーの展開と折りたたみを示していることを確認します。

索引モデルが issue の本文から書いたものです。