[Fix #500] Mark Performance/MapCompact cop as unsafe

[jbpextra]

Performance/MapCompact is marked as safe but it should not be as the suggested refactoring is not equivalent

[true, false, nil].compact              #=> [true, false]
[true, false, nil].filter_map(&:itself) #=> [true]

This is confusing for new Ruby developers.

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• 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).
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• 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.

[koic]

This docs/modules/ROOT/pages/cops_performance.adoc is auto-generated. Can you update the source code document instead? Also, no update to docs/modules/ROOT/pages/cops_performance.adoc is needed.

[jbpextra]

Hi @koic, thanks for the review. I have updated the PR as requested. Cheers.

[koic]

Thanks!