Upgrade to .NET 9 SDK

[mthalman]

This repo should be targeting the .NET 9 SDK from the global.json file. This is a requirement necessary for .NET source build in order to target the net9.0 TFM.

Related to dotnet/source-build#3770

[kapsiR]

Since .NET 7 is going out of support on May 14, 2024, are there any plans to update the TFM to .NET 8 or 9 in the near future? 🤔

Thanks!