ci(devcointainer): add .NET 6.0 on devcontainer

Author: nogic1008

.devcontainer/devcontainer.json

@@ -1,5 +1,5 @@
 {
-  "name": ".NET 10.0 + .NET 9.0 + .NET 8.0",
+  "name": ".NET 10.0 + .NET 9.0 + .NET 8.0 + .NET 6.0",
   "image": "mcr.microsoft.com/vscode/devcontainers/dotnet:9.0",
   "customizations": {
     "vscode": {
@@ -11,7 +11,7 @@
   "features": {
     "ghcr.io/devcontainers/features/dotnet:2": {
       "version": "10.0",
-      "additionalVersions": ["8.0"]
+      "additionalVersions": ["8.0", "6.0"]
     }
   }
 }