Document DefaultValueAttribute constructor DynamicallyAccessedMembers removal breaking change for .NET 10

[Copilot]

This PR documents a breaking change in .NET 10 where the DefaultValueAttribute(System.Type type, string? value) constructor is no longer annotated with DynamicallyAccessedMembersAttribute.

Changes Made

• Created new breaking change documentation file: defaultvalueattribute-dynamically-accessed-members.md
• Added entry to the Core .NET libraries section in the main .NET 10 breaking changes index

Background

The DefaultValueAttribute(System.Type type, string? value) constructor is not supported with trimming and will throw an exception if reached at runtime in a trimmed app. While it was previously possible to disable the exception throwing mechanism with a feature switch, the removal of the DynamicallyAccessedMembers annotation means fewer cases will work reliably.

This is both a binary incompatible and source incompatible breaking change that affects applications using this constructor in trimmed scenarios.

Documentation Details

The documentation follows the established format for breaking changes and includes:

• Clear description of the change and its impact
• Previous vs. new behavior comparison
• Type of breaking change classification
• Reason for the change
• Recommended action for developers
• Affected API reference

Fixes #47444.

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Internal previews

📄 File	🔗 Preview link
docs/core/compatibility/10.0.md	Breaking changes in .NET 10
docs/core/compatibility/core-libraries/10.0/defaultvalueattribute-dynamically-accessed-members.md	DynamicallyAccessedMembers annotation removed from DefaultValueAttribute ctor
docs/core/compatibility/toc.yml	docs/core/compatibility/toc

[BillWagner]

This LGTM as well.