Update templates for platform selection

### Description

[templating/Reference-for-template.json.md at main · dotnet/templating (github.com)](https://github.com/dotnet/templating/blob/main/docs/Reference-for-template.json.md#multichoice-symbols-specifics)

This will enable you to do something like this:

```console
> dotnet new maui -n JustDesktop --Platforms macos windows
```

Including the options will not impact existing template usage from CLI or VS in older versions. When the capability rolls out in .NET 7 and Visual Studio it will light up.

### (Public) API Changes

none

### Usage Scenarios

```console
> dotnet new maui -n JustDesktop --Platforms macos windows
```

For more examples see the docs

### Backward Compatibility

yes, backwards compatible

### Difficulty

Low

Start with docs/Reference-for-template.json.md at the multichoice-symbols section and the `dotnet new maui -n JustDesktop --Platforms macos windows` example. Identify the MAUI template definitions that need platform-selection options, then verify that the command supports the example while existing CLI and Visual Studio usage remains compatible.

Written by the indexing model from the issue text.