:Added practical upgrade lessons learned that may help others

[remcovanvugt]

Overview

I recently encountered a few practical hurdles when upgrading a real old 2.x cluster to 3.x, I guess a few notes in the documentation might help others in the same situation.

Checklist

• [ ] Code is written and works correctly
• [ ] Changes are covered by tests
• [ ] Any new configurable parameters are documented in rel/overlay/etc/default.ini
• [X] Documentation changes were made in the src/docs folder
• [ ] Documentation changes were backported (separated PR) to affected branches