Remove grace period for BackgroundMigration/FeatureCategory 👮‍♂️

What does this MR do and why?

• !106147 (merged) introduced BackgroundMigration::FeatureCategory 👮 with a grace period.
• Since all the existing cases are handled as part of !106147 (merged) and there are no new offenses seen in #f_rubocop in the last ~2 weeks, this MR removes the grace period.
  • Last seen (silenced offense) in #f_rubocop - Jan 6th.

Steps to test locally

RuboCop::Formatter::GracefulFormatter.grace_period? will return false with deleting the .rubocop_todo/background_migration/feature_category.yml config file.

• Didn't test fully in local console, since it needs a new offense to occur.

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.

Closes #386740 (closed)