rubocops: migrate to rubocop v1 API

[jonchang]

See https://docs.rubocop.org/rubocop/v1_upgrade_notes.html for details. Presumably the legacy APIs we were using will go away in Rubocop v2, so let's migrate these now so we don't continue to add new audits that use APIs that will go away one day.

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes? Here's an example.
• Have you successfully run brew style with your changes locally?
• Have you successfully run brew typecheck with your changes locally?
• Have you successfully run brew tests with your changes locally?
• Have you successfully run brew man locally and committed any changes?

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[BrewTestBot]

Review period will end on 2021-01-13 at 08:06:58 UTC.

[MikeMcQuaid]

Thanks @jonchang, good catch here.

[BrewTestBot]

Review period ended.