Bump MSBuild.Sdk.Extras from 2.0.29 to 2.0.31 in /Rx.NET/Source

Bumps [MSBuild.Sdk.Extras](https://github.com/onovotny/MSBuildSdkExtras) from 2.0.29 to 2.0.31.
- [Release notes](https://github.com/onovotny/MSBuildSdkExtras/releases)
- [Commits](https://github.com/onovotny/MSBuildSdkExtras/compare/2.0.29...2.0.31)

Signed-off-by: dependabot-preview[bot] <[email protected]>

Rx.NET/Source/global.json

@@ -3,6 +3,6 @@
     "version": "3.0.100-preview"
   },
   "msbuild-sdks": {
-    "MSBuild.Sdk.Extras": "2.0.29"
+    "MSBuild.Sdk.Extras": "2.0.31"
   }
 }