Add security issue and merge request templates
What does this MR do?
Add security issue and merge request templates
Why was this MR needed?
These templates have slight modifications from https://gitlab.com/gitlab-org/security/gitlab/blob/master/.gitlab/merge_request_templates/Security%20Release.md and https://gitlab.com/gitlab-org/security/gitlab/-/blob/master/.gitlab/issue_templates/Security%20developer%20workflow.md which are templates that have been used by the delivery team for security releases.