RuboCop: Enable previously disabled cop RSpec/ScatteredLet

What does this MR do and why?

This MR enables the following 👮 rule which was disabled only due to too many offenses:

• RSpec/ScatteredLet

Contributes to #369268 (closed).

All pending TODOs have been regenerated via

bundle exec rake rubocop:todo:generate[RSpec/ScatteredLet]

How to set up and validate locally

bundle exec rake rubocop:todo:generate[RSpec/ScatteredLet]

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.