Update feature proposal issue description with solution section as defined in UX guide

What does this MR do?

Update feature proposal issue description template with solution section as defined in https://about.gitlab.com/handbook/engineering/ux/ux-designer/#deliver

What are the relevant issue numbers?

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides