Update Problem Validation issue template

What does this MR do and why?

This MR adds simple but important comments about the goal of the Problem Validation issue template. Today, we use two different problem validation issues: one for the PM to articulate the problem, and another one for the researcher to execute the study. I want to bring clarity in the template description about what each template does.

This proposal also adds a section at the end of the template for linking the validation research item, so that we keep the information and issues always related.

Related to #341548