Bug: Iterations should not auto-close on the due_date, but rather the day after

As reported by user @rbrandon:

We experienced the same issue as @jgutholm: we had iteration 1 with end date August 31st and iteration 2 started on September 1st. On the morning of August 31st, we found out that Iteration 1 had been closed automatically by GitLab. When I tried editing the iteration end date, it told me that I could not put it on september 1st because it would overlap with iteration 2. Setting iteration 2 to start on September 2nd, did not automatically reopen iteration 1.

We're closing iterations on the due_date instead of the day after the due_date.