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/

[microsoft-github-policy-service]

Hi @@JamieMagee. Thank you for your interest in helping to improve the Azure SDK experience and for your contribution. We've noticed that there hasn't been recent engagement on this pull request. If this is still an active work stream, please let us know by pushing some changes or leaving a comment. Otherwise, we'll close this out in 7 days.

[microsoft-github-policy-service]

Hi @@JamieMagee. Thank you for your contribution. Since there hasn't been recent engagement, we're going to close this out. Feel free to respond with a comment containing "/reopen" if you'd like to continue working on these changes. Please be sure to use the command to reopen or remove the "no-recent-activity" label; otherwise, this is likely to be closed again with the next cleanup pass.