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.

This reverts merge request !23381

Automatically adding required approvals to merge reqeusts caused a lot
of notifications. Since that was fixed, we can re-enable CODEOWNERS

We can move the file back to its original location once https://gitlab.com/gitlab-org/gitlab-ee/issues/8396 is fixed.

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

Updates the database changes MR template to match the default template.
Items from the general checklist were removed and a reminder was added
to keep the description up-to-date.

Add ~"test plan" label and update attributes

With https://gitlab.com/gitlab-org/gitlab-ce/issues/50289 we moved CI
templates into gitlab repository.

The previous gitlab-ci-yml maintainers are now marked as code owners.
Auto DevOps is assigned to configure team backend developers.

This ports the changes for the CODEOWNERS feature to CE:
- It adds the CODEOWNERS file.
- It adds the mention of the `with-codeowners` branch in TestEnv