Possible memory leak when reloading images in MarkdownTextBlock

### Describe the bug

Found by Copilot in https://github.com/CommunityToolkit/Labs-Windows/pull/771/

> When using IImageProvider, the _image instance is replaced with a new Image object from the provider. This means the Loaded event handler attached in Init() is lost, since it was attached to the original _image instance. However, since this code is already executing within the LoadImage handler and _loaded is set to true immediately after, this might work correctly. Consider whether the event handler should be detached from the old image or if there's a risk of memory leaks from the orphaned handler on the original image object.

### Steps to reproduce

```text
Unknown, need investigation.
```

### Expected behavior

No memory leaks when images are changed.

### Screenshots

_No response_

### Code Platform

- [ ] UWP
- [ ] WinAppSDK / WinUI 3
- [ ] Web Assembly (WASM)
- [ ] Android
- [ ] iOS
- [ ] MacOS
- [ ] Linux / GTK

### Windows Build Number

- [ ] Windows 10 1809 (Build 17763)
- [ ] Windows 10 1903 (Build 18362)
- [ ] Windows 10 1909 (Build 18363)
- [ ] Windows 10 2004 (Build 19041)
- [ ] Windows 10 20H2 (Build 19042)
- [ ] Windows 10 21H1 (Build 19043)
- [ ] Windows 11 21H2 (Build 22000)
- [ ] Other (specify)

### Other Windows Build number

_No response_

### App minimum and target SDK version

- [ ] Windows 10, version 1809 (Build 17763)
- [ ] Windows 10, version 1903 (Build 18362)
- [ ] Windows 10, version 1909 (Build 18363)
- [ ] Windows 10, version 2004 (Build 19041)
- [ ] Other (specify)

### Other SDK version

_No response_

### Visual Studio Version

_No response_

### Visual Studio Build Number

_No response_

### Device form factor

_No response_

### Additional context

_No response_

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