Consolidate RuboCop rules related to RSpec feature categories

What does this MR do and why?

Unite RSpec/InvalidFeatureCategory and RSpec/MissingFeatureCategory into RSpec/FeatureCategory.

This not only reduces the amount of code but also side-steps potential confusion with RuboCop rule Gemfile/MissingFeatureCategory.

Follow-up of !132679 (merged).

How to set up and validate locally

bundle exec rubocop

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.