Add NOTE callout about DateTime calculation limitations in default interface methods tutorial

[Copilot]

Addresses feedback about DateTime calculation edge cases in the default interface methods tutorial sample code. The tutorial contains DateTime.Now.AddYears(-2) which can have issues with daylight saving time transitions and leap years.

Following maintainer guidance, this change adds a NOTE callout to acknowledge these limitations while keeping the sample code simple and focused on demonstrating default interface methods functionality.

Changes:

• Added NOTE callout after the code snippet explaining DateTime calculation edge cases
• Provides guidance for production code to use UTC time or more robust approaches
• Maintains tutorial simplicity while educating readers about DateTime limitations

The NOTE appears right after the first code example to ensure readers are aware of the considerations before seeing the implementation.

Fixes #26723.

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Internal previews

📄 File	🔗 Preview link
docs/csharp/advanced-topics/interface-implementation/default-interface-methods-versions.md	docs/csharp/advanced-topics/interface-implementation/default-interface-methods-versions

[BillWagner]

This LGTM.

I'll mark it ready for final review.