Update global.json to ensure we're on .NET 6 still for Uno-Check

(Basically, ensure we're on a version of .NET we're expecting in the CI to build with until we want to upgrade.)

Author: michael-hawker

global.json

@@ -1,6 +1,10 @@
 {
-    "msbuild-sdks": 
-    {
-        "MSBuild.Sdk.Extras":"3.0.23"
-    }
-}
+  "sdk": {
+    "version": "6.0.403",
+    "rollForward": "latestFeature"
+  },
+  "msbuild-sdks": 
+  {
+    "MSBuild.Sdk.Extras":"3.0.23"
+  }
+}