Adjust security merge request template

What does this MR do?

Adjust the changelog step on the security merge request template.

The new changelog process has been rolled out gitlab-com/gl-infra/delivery#1564 (closed). This commit updates the security release MR template to use this new process instead of generating an entry on changelogs/unreleased

TODO

• [-] Added the Changelog (e.g. Changelog: feature) trailer to the commits that need to be included in the changelog
• [-] Added unit tests (if required)
• [-] Added acceptance tests (if required)
• [-] Documentation (if required)
• Code review guidelines
• [-] Go Style guidelines