Create security release MR template
What does this MR do?
This MR adds a new merge request template to be used for security fixes MRs. It also removes some duplicate information about security process for developers that is spread between docs and templates.
Related issues
Closes https://gitlab.com/gitlab-org/release/framework/issues/167
Closes https://gitlab.com/gitlab-org/release/framework/issues/166
Closes https://gitlab.com/gitlab-org/release/framework/issues/168
Related to https://gitlab.com/gitlab-org/release/framework/issues/162