[flutter_markdown] Replace deprecated `renderViewElement`

```
'renderViewElement' is deprecated and shouldn't be used. Use rootElement instead.
This feature was deprecated after v3.9.0-16.0.pre. Try replacing the use of the deprecated member with the
replacement.
```

This was just waiting for 3.9 as a minimum version, so should be straightforward now. It's also in test-only code.