This makes two changes to the template:

1. EE MRs are now always required (see
   https://gitlab.com/gitlab-org/release/framework/issues/256 for a
   backstory).

2. We clarify that a release manager is not a suitable reviewer for
   security merge requests.

In https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24962 the
stable branches were incorrectly changed to `stable-X-Y`. This commit
fixes this so we correctly use `X-Y-stable`.

This removes the requirement and any mention of targeting security
branches when working on security releases. The release process
documentation changes for these CE changes can be found in merge request
https://gitlab.com/gitlab-org/release/docs/merge_requests/97.

The proposal to remove security branches was approved in
https://gitlab.com/gitlab-org/release/framework/issues/165#note_138139016.

Improve existing issue templates for security releases