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.

Because there were a few people thought that's for UI styling...

Improve existing issue templates for security releases

Update issue template with user journey, persona label additions, and clarification between success metrics and acceptance criteria

just ~feature.

It now asks to assign the main dev issue to all RMs

Add ~"test plan" label and update attributes

The issue template for bugs has a notice at the start that mentions to
search for duplicates before creating a new issue. Then the reporter
is asked to remove the notice once read, presumptively to avoid noise
in the actual issue.

Instead of that, we can make that part of the issue template an html
comment, so it won't be rendered in the final result. This avoids
the tedious task of removing that part of the notice.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/43580

[skip ci]