[Fix #10202] Add new Lint/Ruby2Keywords cop

[dvandersluis]

Looks for instances of ruby2_keywords for methods that do not need it.

ruby2_keywords is only needed for methods that have a splat (*args) and no keyword args (regular, optional or keyword splat).

Can handle three forms:

ruby2_keywords def foo(arg)
end

def foo(arg)
end
ruby2_keywords :foo

define_method(:foo) { |arg| }
ruby2_keywords :foo

Fixes #10202.

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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.

[koic]

I think the cop name UselessRuby2Keywords cop is better.

[dvandersluis]

@koic I renamed the cop and changed the examples. I still think it's not a bad thing to have separate @example blocks for different use cases (I understand that we usually do this for configuration examples, but still), but I rewrote it to be more like you were suggesting.