This PR adds obsolete attributes to three MSTest APIs that will be removed in v4.0, providing early feedback to users in v3.10.

Changes Made

Added [Obsolete] attributes to:

• CssIterationAttribute - CSS Iteration URI attribute
• CssProjectStructureAttribute - CSS Project Structure URI attribute
• DescriptionAttribute - Description of the test attribute

The obsolete attributes follow the existing pattern used by other deprecated types in the codebase:

#if NET6_0_OR_GREATER
[Obsolete(FrameworkConstants.PublicTypeObsoleteMessage, DiagnosticId = "MSTESTOBS")]
#else
[Obsolete(FrameworkConstants.PublicTypeObsoleteMessage)]
#endif

User Impact

When users compile code using these attributes in v3.10, they will see warnings:

Warning: 'CssIterationAttribute' is obsolete: 'We will remove or hide this type starting with v4. If you are using this type, reach out to our team on https://github.com/microsoft/testfx.'

This provides time for users to migrate away from these attributes before they are completely removed in v4.0 (as done in PR #5858).

Technical Details

• No breaking changes - attributes remain functional in v3.10
• Minimal implementation - only added 15 lines total (5 per file)
• Follows existing patterns - uses standard PublicTypeObsoleteMessage constant
• Conditional compilation - includes DiagnosticId for NET6_0_OR_GREATER

Fixes #5980.

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