Fix a false negative for `FactoryBot/AssociationStyle` when using keyword traits with explicit associations

[r7kamura]

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A change related to keywords was introduced here:

• https://github.com/rubocop/rubocop-factory_bot/pull/60

At that time, the implementation was adjusted so that the following code would not be treated as an offense, but shouldn’t this actually be considered an offense?

# bad
association :foo, :alias

# good
foo factory: %i[foo alias]

In addition to changing the implementation, I made a few improvements:

1. Converted KEYWORDS from an Array to a Set to improve performance.
2. Updated the code so that only correctable explicit associations are treated as offenses, making the intention clearer.

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Before submitting the PR make sure the following are checked:

• [x] Feature branch is up-to-date with master (if not - rebase it).
• [x] Squashed related commits together.
• [x] Added tests.
• [x] Updated documentation.
• [x] Added an entry to the CHANGELOG.md if the new code introduces user-observable changes.
• [x] The build (bundle exec rake) passes (be sure to run this locally, since it may produce updated documentation that you will need to commit).

If you have created a new cop:

• [ ] Added the new cop to config/default.yml.
• [ ] The cop is configured as Enabled: pending in config/default.yml.
• [ ] The cop documents examples of good and bad code.
• [ ] The tests assert both that bad code is reported and that good code is not reported.
• [ ] Set VersionAdded: "<<next>>" in default/config.yml.

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• [ ] Set VersionChanged: "<<next>>" in config/default.yml.