Update release-candidates doc

This updates the release-candidates doc to say the automatic release candidate creation has been disabled, linking to the relevant issue.

The issue has been open for over 2 years, which makes me suspect automatic RCs have been disabled for some time now and it makes sense to list this in the docs.

I also updated the reference to the template at the bottom to point to the correct section.

Related to gitlab-com/gl-infra/delivery#2790 (closed)