Remove threat_monitoring from GraphQL enum for alerts

What does this MR do and why?

This MR deprecates threat_monitoring in GraphQL enum for alerts as it is removed from %15.0 release.

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 #352287 (closed)