Fix sample projects to work in GitHub Codespaces by installing .NET 9.0 SDK (#854)

* Initial plan

* Update global.json to use .NET 9.0 SDK for GitHub Codespaces compatibility

Co-authored-by: eiriktsarpalis <2813363+eiriktsarpalis@users.noreply.github.com>

* Revert global.json and update devcontainer.json to install both .NET 9 and 10

Co-authored-by: eiriktsarpalis <2813363+eiriktsarpalis@users.noreply.github.com>

* Fix devcontainer.json to use additionalVersions for multiple .NET SDKs

Co-authored-by: eiriktsarpalis <2813363+eiriktsarpalis@users.noreply.github.com>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: eiriktsarpalis <2813363+eiriktsarpalis@users.noreply.github.com>

Author: Copilot

.devcontainer/devcontainer.json

@@ -3,7 +3,8 @@
 	"image": "mcr.microsoft.com/devcontainers/dotnet:1-8.0-jammy",
 	"features": {
 		"ghcr.io/devcontainers/features/dotnet:2": {
-			"version": "10.0"
+			"version": "10.0",
+			"additionalVersions": "9.0"
 		},
 		"ghcr.io/devcontainers/features/node:1": {}
 	},