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/

[Copilot]

Pull Request Overview

Adds a Dependabot configuration entry to monitor and update the .NET SDK version defined in global.json on a weekly schedule, while ignoring major and minor updates.

• Introduces a dotnet-sdk package-ecosystem block under updates
• Configures weekly scans every Wednesday
• Ignores semver-major and semver-minor updates (allowing only patch updates)

Comments suppressed due to low confidence (2)

.github/dependabot.yml:16

• Ensure this new - package-ecosystem entry is indented to match the other updates: list items exactly; misaligned YAML sequence markers can cause parse errors.

  - package-ecosystem: dotnet-sdk

.github/dependabot.yml:26

• [nitpick] Consider removing the leftover commented-out stub lines (26–28) if they’re no longer needed, to keep the configuration file clean and focused.

#- package-ecosystem: dotnet-sdk