Add Dependabot configuration for .NET SDK updates

[JamieMagee]

This PR adds Dependabot configuration to automatically manage .NET SDK version updates in global.json.

This configuration will:

• Monitor the global.json file for .NET SDK version updates
• Create pull requests when new SDK versions are available
• Help keep the project secure with the latest SDK patches

For more information, see: https://devblogs.microsoft.com/dotnet/using-dependabot-to-manage-dotnet-sdk-updates/