Update security engineer docs
What does this MR do?
Update security engineer process
Starting with the regular security release of %13.2, security merge requests targeting 'master' will require approval of AppSec to be considered in a security release.
AppSec engineers will validate the security fix using the image generated by the package-and-qa build. This commit updates the documentation of the security engineer
Related to gitlab-com/gl-infra/delivery#839 (closed)