Document support for Android 6.0 (API 23) or higher for MAUI Blazor apps

The [Supported platforms](https://github.com/dotnet/docs-maui/blob/main/docs/supported-platforms.md) page specifies Android API 21 as the minimum supported Android API level for MAUI apps. However, MAUI Blazor apps specifically require at minimum Android API level 23. This caveat should be documented.

See: https://github.com/dotnet/maui/issues/3023