Reduce confusion in issue creation template about the section for meeting planners

[ianbjacobs]

This heading is creating some confusion: "Scheduling conflicts to avoid (For meeting planners only)"

I think it's not clear enough that this is a private section for the meeting planners. I propose that (after TPAC 2025)
we change the code and template so that the heading is "Do not edit below this line (For meeting planners only)"

It won't work 100% of the time but it may help.