Add deprecation alpha to the work items API

What does this MR do and why?

When the feature flag is enabled by default we don't want users to start using the work items API. We can remove this warning once the API is more stable.

Related thread in !87741 (comment 958722696)

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #334809 (closed)