Add step to security dev workflow template

What does this MR do and why?

This adds a step to the security developer workflow issue template to add the security-target label. This label will be used in the tooling being built in gitlab-com/gl-infra&1061 (closed) for automating some of the security release process.

The step in this template will be updated when we roll out the changes to reflect that it is required in the future when it replaces the step above it.

Related to gitlab-com/gl-infra/delivery#19634 (closed)

Screenshots or screen recordings

n/a

How to set up and validate locally

n/a

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.