Pin .NET Core SDK to 3.1.*

Pin the .NET Core SDK version to 3.1.*, not allowing prerelease
versions.

Author: Ralph Hendriks

global.json

@@ -1,5 +1,7 @@
 {
   "sdk": {
-    "version": "3.1.201"
+    "allowPrerelease": false,
+    "version": "3.1.100",
+    "rollForward": "latestFeature"
   }
 }