Fix Rails/CompactBlank to avoid reporting offense for filter in Ruby versions below 2.6

[masato-bkn]

This PR fixes Rails/CompactBlank to avoid reporting offense for filter in Ruby versions below 2.6.

This change is related to #1359. Apologies for the confusion earlier, as I initially mixed up the runtime version with the analysis version when considering Rubocop's supported versions.

Rails/CompactBlank applies to Rails 6.1 and above, where the required Ruby version is 2.5. However, since filter method was introduced in Ruby 2.6, using filter in environments running Ruby 2.5 may result in false positives.

Additionally, since #1359 is still unreleased, I am unsure whether this change should be added to the changelog. I would appreciate any feedback, including thoughts on other aspects of this PR.

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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.
  * [] If this is a new cop, consider making a corresponding update to the [Rails Style Guide](https://github.com/rubocop/rails-style-guide).

[koic]

Thank you for following up!