Enable `dast_api_scanner` feature flag

Related issues

• Feature flag rollout issue - #345837 (closed)
• Documentation issue - #352625 (closed)

Blocked by #378325 (closed).

What does this MR do and why?

Enable dast_api_scanner feature flag with documentation updates

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.