Adjust changelog step on security MR 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)

Does this MR meet the acceptance criteria?

Conformity

• [-] I have included a changelog entry, or it's not needed. (Does this MR need a changelog?)
• [-] I have added/updated documentation, or it's not needed. (Is documentation required?)
• [-] I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• [-] I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.