Use Ruby 2.7 parser for rake documentation_syntax_check

[koic]

Summary

This PR uses Ruby 2.7 parser for rake documentation_syntax_check.
With this change, example code written using Ruby 2.7 syntax can be analyzed.

Other infromation

The following error occurs in Lint/CircularArgumentReference example code when Ruby 2.7 parser is used.

% bundle exec rake documentation_syntax_check
Files:         474
Modules:        83 (   11 undocumented)
Classes:       438 (    3 undocumented)
Constants:     631 (  621 undocumented)
Attributes:     31 (    0 undocumented)
Methods:      1081 (  960 undocumented)
 29.55% documented
 lib/rubocop/cop/lint/circular_argument_reference.rb: Syntax Error in an
 example. circular argument reference pie
 lib/rubocop/cop/lint/circular_argument_reference.rb: Syntax Error in an
 example. circular argument reference dry_ingredients

The error is avoided by using Ruby 2.6 parser for Lint/CircularArgumentReference example code as a workaround.

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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.
• 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.

[bbatsov]

Thanks!