[Fix #384] Support optimized `String#dup` for `Performance/UnfreezeString`

[koic]

Resolves #384.

This PR supports optimized String#dup for Performance/UnfreezeString when Ruby 3.3+.

To incorporate https://github.com/rubocop/rubocop/commit/d11e25f the RuboCop dependency will be updated to 1.48.1+ from 1.30.0+, but the supported runtime Ruby version will keep at Ruby 2.6+.

• https://rubygems.org/gems/rubocop/versions/1.30.0
• https://rubygems.org/gems/rubocop/versions/1.48.1

---

Before submitting the PR make sure the following are checked:

• [x] The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• [x] Wrote good commit messages.
• [x] Commit message starts with [Fix #issue-number] (if the related issue exists).
• [x] Feature branch is up-to-date with master (if not - rebase it).
• [x] Squashed related commits together.
• [x] Added tests.
• [x] Ran bundle exec rake default. It executes all tests and runs RuboCop on its own code.
• [x] 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.