Add WARNING headers to API responses that have deprecated features

We include deprecation information in documentation, but we can help clients to notice deprecations by using the warning header standard.

Release notes

Problem to solve

Users may write and deploy API integrations, and not notice that some features later get deprecated. It is unrealistic to expect users and developers to keep up-to-date with all deprecation notices.

Proposal

For all API (REST and GraphQL) responses that include a deprecated feature, add an appropriate warning header including a link to the deprecation notice with full details.

Intended users

Feature Usage Metrics

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