Update solution validation steps

Why is this change being made?

Follow up to the discussion to modify the solution validation process.

Individuals responsible for each step have changed with the shift to having researchers participate in problem validation and other strategic efforts. With this, Product Designers and Product Design Managers will fill in.

Note: Additional MRs will be needed to remove inconsistencies and duplications elsewhere in the handbook. The goal is to have the Product Development Flow be the SSOT.