Bump to use .NET 9 SDK

[JoshLove-msft]

We should use .NET 9 SDK matching azure-sdk-for-net.

[ArcturusZhang]

would this bring up a requirement for plugin writers to update as well?

[JoshLove-msft]

would this bring up a requirement for plugin writers to update as well?

No, I don't think it would unless the plugin lived in this repo.

[JoshLove-msft]

Post in the generator channel when making this change so everyone is aware as it will require installing the .NET 9 SDK.