Do not update `updated_at` on an issue when reordering it

Description

Per https://gitlab.com/gitlab-org/gitlab-ce/issues/39251#note_66200000:

• Do not change updated_at on an issue when changing positions in a list on an issue board.
• Do not change updated_at on an issue when changing positions in a list on an epic.
• In particular, when you move an issue in a list, no updated_at values for any issues should change as a result. Not for that issue, and not for any other issues in that list.
• After this issue is complete, we can re-visit the ordering as mentioned in https://gitlab.com/gitlab-org/gitlab-ce/issues/39251.

Background

If we interpret updated_at as a timestamp reflecting user-facing changes in the issue itself, then this attribute should not change with reordering issues. Reordering issues in a list reflects the relative order between issues. It is not inherent to the issue itself. So we can thus interpret that as not changing the issue from a user-perspective.