[Fix #11399] Add new `Style/UnlessMinimizeNegations` cop

[SparLaimor]

Fixes #11399. This PR adds new Style/UnlessMinimizeNegations cop.

It minimizes the number of negations in an unless using De Morgan’s laws.

# bad (all terms negated)
do_something unless !x && !y

# good (all terms positive)
do_something if x || y

# bad (2 negations, 1 positive)
do_something unless !x || !y || z

# good (2 positives, 1 negation)
do_something if x && y && !z

---

Before submitting the PR make sure the following are checked:

• [x] The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• [x] Wrote good commit messages.
• [x] Commit message starts with [Fix #issue-number] (if the related issue exists).
• [x] Feature branch is up-to-date with master (if not - rebase it).
• [x] Squashed related commits together.
• [x] Added tests.
• [x] Ran bundle exec rake default. It executes all tests and runs RuboCop on its own code.
• [x] 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.

[bbatsov]

@SparLaimor friendly ping :-)

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