[Fix #1199] Make Rails/WhereEquals aware of where.not(...)

[Earlopain]

Fix #1199

This copies a bit of logic from WhereRange but they need slighly different handling and it doesn't seem worth to extract yet.

---

Before submitting the PR make sure the following are checked:

• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Ran bundle exec rake default. It executes all tests and runs RuboCop on its own code.
• Added an entry (file) to the changelog folder named {change_type}_{change_description}.md if the new code introduces user-observable changes. See changelog entry format for details.
• If this is a new cop, consider making a corresponding update to the Rails Style Guide.

[koic]

Thanks!

[Earlopain]

@koic, the changelog type should have been change, right? I feel like that would be more appropriate. If you feel the same, just fix in main. Thanks!