Add .NET 7.0 SDK feature to devcontainer configuration (microsoft#1424)

### Motivation and Context
Resolves microsoft#1421 by specifying .NET 7.0 SDK as feature for dev container
to install.

Co-authored-by: Craig Presti <[email protected]>

Author: pjirsa

.devcontainer/devcontainer.json

@@ -2,7 +2,9 @@
 	"image": "mcr.microsoft.com/devcontainers/universal:2",
 	"features": {
 		"ghcr.io/devcontainers/features/node:1": {},
-		"ghcr.io/devcontainers/features/dotnet:1": {},
+		"ghcr.io/devcontainers/features/dotnet:1": {
+			"version": "7"
+		},
 		"ghcr.io/jlaundry/devcontainer-features/azure-functions-core-tools:1": {}
 	},
 	"customizations": {