Add .NET 9 support, remove .NET 6

[jasongin]

• Update global.json to .NET 9 instead of .NET 8.
• Replace the .NET 6 target everywhere with .NET 9
  • Keep supporting .NET 8 and .NET Framework 4.7.2
  • .NET 8 is still the default for some things since 8 is LTS and 9 is not.
• Fix some minor compatibility issues with .NET 9 SDK.
• Use Node.js v22 instead of v20 in build pipelines.
  • Keep testing with Node.js v18 also.
• Update package version from 0.8 to 0.9

At this point all the tests are passing, but I want to make sure the example projects are in good shape too before checking this in.