Update blog post link in issue template

What does this MR do?

Update blog post link in issue template

Draf of the Security blog post is now performed in https://gitlab.com/gitlab-org/security/www-gitlab-com, this MR updates the link in the template

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