Enable RuboCop Style/ClassAndModuleChildren

What does this MR do and why?

This MR enables 👮 Style/ClassAndModuleChildren and excludes current offenses via

bundle exec rake rubocop:todo:generate[Style/ClassAndModuleChildren]

See #39044 (closed)

Screenshots or screen recordings

n/a

How to set up and validate locally

REVEAL_RUBOCOP_TODO=0 bundle exec rubocop

rubocop --show-cops

Style/ClassAndModuleChildren:
  Description: Checks style of children classes and modules.
  StyleGuide: "#namespace-definition"
  SafeAutoCorrect: false
  Enabled: true
  VersionAdded: '0.19'
  EnforcedStyle: nested
  SupportedStyles:
  - nested
  - compact

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.