Updates security templates for `12-8-stable-ee`

What does this MR do?

Updates the security issue and merge request templates from the ones we have on master. This is needed to reflect the new security setup

Does this MR meet the acceptance criteria?

Conformity

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

Related gitlab-com/gl-infra/delivery#689 (closed)