Remove unnecessary sections from Lean template

What does this MR do and why?

This MR removes the following unnecessary sections from the Lean template:

• extra Intended Users section from the Feature Proposal - lean.md template that was mistakenly added in Adds intended users and metrics to Basic template per feedback issue
• User experience goal
• Further details
• Permissions and Security
• Documentation
• Availability & Testing
• What does success look like, and how can we measure that?
• What is the type of buyer?
• Is this a cross-stage feature?

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