Update devcontainer

Updates devcontainer to 8.0 tag (not preview), and also gets the runtime feature for the VSCode extension to pre-load it

Author: timheuer

.devcontainer/devcontainer.json

@@ -1,6 +1,6 @@
 {
     "name": ".NET in Codespaces",
-    "image": "mcr.microsoft.com/dotnet/sdk:8.0-preview",
+    "image": "mcr.microsoft.com/dotnet/sdk:8.0",
     "features": {
         "ghcr.io/devcontainers/features/docker-in-docker:2": {},
         "ghcr.io/devcontainers/features/github-cli:1": {
@@ -12,7 +12,12 @@
         "ghcr.io/azure/azure-dev/azd:0": {
             "version": "latest"
         },
-        "ghcr.io/devcontainers/features/common-utils:2": {}
+        "ghcr.io/devcontainers/features/common-utils:2": {},
+        "ghcr.io/devcontainers/features/dotnet:2": {
+            "version": "none",
+            "dotnetRuntimeVersions": "7.0",
+            "aspNetCoreRuntimeVersions": "7.0"
+        }
     },
     "customizations": {
         "vscode": {