Remove .NET SDK pin from global.json

CONTRIBUTING.md

@@ -1,6 +1,20 @@
 
 # Contributing
 
+## Prerequisites
+
+This repository does not pin a specific .NET SDK version in `global.json`; the
+build uses whichever SDK is installed. To build and test locally you need:
+
+- A current .NET 10 SDK (the build targets the latest 10.x).
+- .NET 8 and .NET 9 SDKs as well, since the test projects target `net8.0`,
+  `net9.0`, and `net10.0` (plus `net472` on Windows).
+
+CI installs these via `UseDotNet@2` (`8.x`, `9.x`, `10.x`); mirror that
+locally. Download them from <https://dotnet.microsoft.com/download>.
+
+## Contributions
+
 This project welcomes contributions and suggestions.  Most contributions require you to agree to a
 Contributor License Agreement (CLA) declaring that you have the right to, and actually do, grant us
 the rights to use your contribution. For details, visit https://cla.microsoft.com.

global.json

@@ -1,9 +1,4 @@
 {
-  "sdk": {
-    "version": "10.0.203",
-    "rollForward": "latestMinor",
-    "allowPrerelease": true
-  },
   "msbuild-sdks": {
     "Microsoft.Build.NoTargets": "3.7.134"
   }