Update security release template for the new setup

What does this MR do?

We now require that security issues are marked as related, instead of being added to the issue body.

See gitlab-com/gl-infra/delivery#689 (closed) for more information.

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content