Rubocop: disable pending cops

This avoids having the output polluted with:

  The following cops were added to RuboCop, but are not configured. Please set Enabled to either `true` or `false` in your `.rubocop.yml` file:
   - Layout/EmptyLinesAroundAttributeAccessor (0.83)
   - Layout/SpaceAroundMethodCallOperator (0.82)
   - [… dozens more cops …]

These new cops will be enabled in the next Rubocop major release.

For details, see https://docs.rubocop.org/rubocop/versioning.html