Remove guidance on closing issues that are not the next iteration

We need to be aware that GitLab employees are not the only users of our issue trackers, our broader community participates as well. Closing issues that GitLab believes to be "not the next iteration", could easily cause frustration with the broader community, as well as lose potentially valuable feedback. For example, closed issues are typically much harder to find when searching, which means duplicate issues may be created with lost context.

We need to ensure we are not optimizing purely for GitLab employees: https://about.gitlab.com/handbook/values/#efficiency-for-the-right-group

This change leaves other important reasons to close issues, but removes the guidance on closing anything beyond the next iteration.