Deprecate promoted_issues metric

What does this MR do and why?

This metric is no longer needed and was introduced by !40024 (merged).

See deprecation process: https://docs.gitlab.com/ee/development/service_ping/metrics_lifecycle.html#service-ping-metric-lifecycle

Related to #342161 (closed)

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.