Add <> operator to Rails/WhereNot cop.

[Tietew]

SQL standard defines <> as not-equal operator. != is a de facto standard.

cf. https://www.postgresql.org/docs/12/functions-comparison.html
(I'm afraid I have no pointer to SQL standard documentation.)

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Before submitting the PR make sure the following are checked:

• 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.
• Added an entry to the Changelog if the new code introduces user-observable changes. See changelog entry format.
• If this is a new cop, consider making a corresponding update to the Rails Style Guide.
• The PR relates to only one subject with a clear title
  and description in grammatically correct, complete sentences.
• Run bundle exec rake default. It executes all tests and RuboCop for itself, and generates the documentation.

[koic]

@Tietew This looks good to me. Can you rebase with the latest master branch?

[Tietew]

@koic Rebased!

[koic]

@Tietew Thank you!