Fix an error for Minitst/AssertOperator and Minitest/RefuteOperator

[koic]

This PR fixes the following error for Minitst/AssertOperator and Minitest/RefuteOperator
when running with Ruby 2.7:

$ ruby -v
ruby 2.7.8p225 (2023-03-30 revision 1f4d455848) [x86_64-darwin19]

$ bundle exec rake

Finished in 2.522134s, 15.0666 runs/s, 18.6350 assertions/s.

  1) Error: RefuteOperatorTest#test_registers_offense_when_using_refute_with_operator_method_and_message:
FrozenError: can't modify frozen String: "expected, :<, actual"
    /Users/koic/src/github.com/rubocop/rubocop-minitest/lib/rubocop/cop/minitest/refute_operator.rb:46:
    in `build_new_arguments'
    /Users/koic/src/github.com/rubocop/rubocop-minitest/lib/rubocop/cop/minitest/refute_operator.rb:30:
    in `on_send'

And it introduces CI for the oldest RuboCop version still supported, implementing a workflow for regression testing that is similar to the one found a rubocop/rubocop-rails@c9acb7a.

---

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.