Fix the devcontainers dotnet feature version (#488)

Author: TechWatching

.devcontainer/devcontainer.json

@@ -5,7 +5,7 @@
     "ghcr.io/azure/azure-dev/azd:latest": {},
     "ghcr.io/devcontainers/features/dotnet:latest": {
       "version": "8.0",
-      "additionalVersions": ["9.0"]
+      "additionalVersions": "9.0"
     },
     "ghcr.io/devcontainers/features/github-cli:latest": {},
     "ghcr.io/devcontainers/features/java:1": {