Update 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
Related to gitlab-com/gl-infra/delivery#1767 (closed)