Move to RTM sdk (#2473)

Pull in timestamp fixes

Author: mmitche

global.json

@@ -1,6 +1,6 @@
 {
   "tools": {
-    "dotnet": "3.1.100-preview1-014024",
+    "dotnet": "3.1.100",
     "vs": {
       "version": "16.0"
     },
@@ -12,7 +12,7 @@
     }
   },
   "sdk": {
-    "version": "3.1.100-preview1-014024"
+    "version": "3.1.100"
   },
   "msbuild-sdks": {
     "Microsoft.DotNet.Arcade.Sdk": "1.0.0-beta.19577.5",