Pin .NET Core SDK version to latest feature band

Pin the .NET Core SDK version to the `3.1.1xx` feature band. Do not
allow prerelease versions.

Fixes #175

Author: ralphhendriks

global.json

@@ -1,5 +1,7 @@
 {
   "sdk": {
-    "version": "3.1.102"
+    "allowPrerelease": false,
+    "version": "3.1.100",
+    "rollForward": "latestPatch"
   }
 }