Updates security templates for `12-7-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

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)