Problem: Documentation does not specify how to upgrade if several versions behind the current release

The Upgrading documentation does not give the advice that we recommend upgrading one version at a time (e.g. From AtoM 2.7 to 2.8 and from 2.8 to 2.9). rather than skipping over major version (e.g. 2.6 to 2.9).

It would be useful to clarify this advice in the upgrading docs to reduce community upgrade issues.

[Upgrading docs](https://www.accesstomemory.org/en/docs/2.9/admin-manual/installation/upgrading/)

Start with the linked Upgrading documentation page and review the existing upgrade guidance. Clarify that users should upgrade one version at a time, including the AtoM 2.7 to 2.8 to 2.9 example, and consider the work done when the published instructions address upgrades that span several releases.

Written by the indexing model from the issue text.