[Feature Request] conditionally target .NET9-preview

[jennyf19]

see this .NET8-preview PR as an example of what to do

https://github.com/AzureAD/microsoft-identity-web/pull/2318/files

Make sure the .NET9-preview is added to the GitHub action and the right builds. We should also have a build variable for .NET 9

start with 9.0.100-preview.2.24157.14
https://dotnet.microsoft.com/en-us/download/dotnet/9.0 and we'll update it every month. Using a * does not work well in this case.

[jmprieur]

An even better condition to add the new target framework is to modify <TargetFrameworks> conditionally (adding the line in green below)

<TargetFrameworks>net6.0; net7.0; net8.0; net462; net472; netstandard2.0</TargetFrameworks>
+ <TargetFrameworks Condition="'$(TargetNet9)'== 'True'">$(TargetFrameworks); net9.0</TargetFrameworks>