Use csharp dev kit in Codespaces (#4557)

.devcontainer/devcontainer.json

@@ -17,7 +17,7 @@
         "vscode": {
             "extensions": [
                 "ms-vscode.cpptools",
-                "ms-dotnettools.csharp",
+                "ms-dotnettools.csdevkit",
                 "EditorConfig.EditorConfig",
                 "ms-vscode.powershell",
                 "tintoy.msbuild-project-tools",

.devcontainer/musl/devcontainer.json

@@ -17,7 +17,7 @@
         "vscode": {
             "extensions": [
                 "ms-vscode.cpptools",
-                "ms-dotnettools.csharp",
+                "ms-dotnettools.csdevkit",
                 "EditorConfig.EditorConfig",
                 "tintoy.msbuild-project-tools",
                 "streetsidesoftware.code-spell-checker",

.vscode/settings.json

@@ -19,15 +19,15 @@
     // ms-dotnettools.csharp settings
     "csharp.format.enable": true,
     "csharp.semanticHighlighting.enabled": true,
+    "dotnet.defaultSolution": "dotnet-monitor.sln",
+    "dotnet.completion.showCompletionItemsFromUnimportedNamespaces": true,
 
     // ms-dotnettools.csharp settings
     "omnisharp.path": "latest",
-    "omnisharp.defaultLaunchSolution": "dotnet-monitor.sln",
     "omnisharp.disableMSBuildDiagnosticWarning": true,
     "omnisharp.useModernNet": true,
     "omnisharp.enableAsyncCompletion": true,
     "omnisharp.enableEditorConfigSupport": true,
-    "omnisharp.enableImportCompletion": true,
     "omnisharp.enableRoslynAnalyzers": true,
     "omnisharp.organizeImportsOnFormat": true,
     "omnisharp.autoStart": true,