Update experiment implementation template

What does this MR do and why?

Updates to the experiment implementation template to streamline issue description completion

Specifically removing the Segmentation and Inclusion/Exclusion section and replacing it with Target Population as a result of #416756 (closed)

Screenshots or screen recordings

Issue template description update

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.