Update global.json to require minimum .NET SDK 8.0.400

- Changed from 8.0.100 to 8.0.400 to ensure CI environment uses compatible SDK version
- This should resolve Microsoft.NET.ILLink.Tasks version conflicts (8.0.17 vs 8.0.10)
- CI will now use SDK that includes the same package versions as local environment

Author: StevenTCramer

global.json

@@ -1,6 +1,6 @@
 {
   "sdk": {
-    "version": "8.0.100",
+    "version": "8.0.400",
     "rollForward": "latestFeature",
     "allowPrerelease": false
   }