Deprecate the old format of trailers in the API, to be replaced with the format of extended_trailers

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Summary

This issue tracks the deprecation of the old trailers field format in the GitLab API, which will be replaced by the extended_trailers field format in a future release.

Background

The extended_trailers field has already been implemented and provides enhanced functionality compared to the legacy trailers field. To prepare for a future breaking change, we need to mark the old trailers field as deprecated while maintaining backward compatibility.

Tasks

• Mark the trailers field as deprecated in the API code
• Update API documentation to indicate trailers field deprecation
• Add deprecation notices pointing users to use extended_trailers instead
• Ensure both fields continue to work during the deprecation period

Related Issues

This is part of the broader effort tracked in &12268

Related workstreams:

• #435557 (closed)
• #435558 (closed)

Acceptance Criteria

• The trailers field is marked as deprecated in code with appropriate deprecation warnings
• API documentation clearly indicates the deprecation and migration path to extended_trailers
• Existing functionality remains unchanged during deprecation period
• Users receive clear guidance on migrating to extended_trailers