Update the minimum required .NET SDK to version 5.0.103

Already using LangVersion = 9.0 so the .NET SDK 5 is a requirement.

Also change the roll-forward policy from latestMajor to latestMinor to make sure not to automatically opt-in to the .NET 6 SDK before making sure everything works fine with the .NET 6 SDK.

Author: 0xced

global.json

@@ -1,8 +1,8 @@
 {
   "$schema": "https://json.schemastore.org/global",
   "sdk": {
-    "version": "3.1.405",
+    "version": "5.0.103",
     "allowPrerelease": false,
-    "rollForward": "latestMajor"
+    "rollForward": "latestMinor"
   }
 }