[feature request] Add support for .NET 9

[xiang17]

Component

None

Is your feature request related to a problem?

No response

What is the expected behavior?

Creating an issue to track adding .NET 9 SDK to CI and tests since it's growing to have more sub tasks. Individual component owners can add support for .NET 9 at their own pace.

• Add .NET 9 SDK to CI to build existing project
• Adding .NET 9 has introduced a ton of syntax suggestions showing as errors
  • Setting AnalysisLevel to 8.0 for now to minimize changes in my PR: [repo] Bump SDK to 9.0.100 #2316 (comment)
  • @Kielek has been also working on another PR [repo/examples] Prepare to .NET9 #2251 as the effort to get the code to follow latest analyzer recommendations
• .NET 9 stable SDK 9.0.100 is not available yet in mcr.microsoft.com/dotnet/sdk:9.0 docker image. global.json will need to stay to 9.0.100-rc.2 version as a result for now: [repo] Bump SDK to 9.0.100 #2316 (comment)
• Geneva exporter has two failing unit test cases after adding .NET 9 in CI (the target framework is still net8.0): [repo] Bump SDK to 9.0.100 #2316 (comment)
• Support AOT Compatibility test for .NET 9: Addressed in [repo] Bump SDK to 9.0.100 #2316.

Which alternative solutions or features have you considered?

N/A

Additional context

No response

[xiang17]

Closing this issue because the only remaining task is Geneva exporter's failing test cases, which is tracked in #2326.