ci(devcontainer): add .NET 8 on devcontainer

use workaround of devcontainers/spec#44

Author: nogic1008

.devcontainer/devcontainer.json

@@ -12,6 +12,10 @@
     "ghcr.io/devcontainers/features/dotnet:1": {
       "version": "6"
     },
+    "ghcr.io/devcontainers/features/dotnet:1.1": {
+      "version": "8.0",
+      "installUsingApt": false
+    },
     "ghcr.io/rocker-org/devcontainer-features/apt-packages:1": {
       "packages": "mono-complete"
     }