Build: roll-forward .NET Core SDK to feature (instead of latestPatch)

The change doesn't affect CI or those with the chosen SDK installed, but unbreaks those affected by the VisualStudio installer automatically uninstalling older feature versions. #698 #699

global.json

@@ -1,6 +1,6 @@
 {
     "sdk": {
         "version": "3.1.101",
-        "rollForward": "latestPatch"
+        "rollForward": "feature"
     }
 }