[Fix #10134] Update Style/MutableConstant to not consider multiline uninterpolated strings as unfrozen in ruby 3.0

[dvandersluis]

In #10006 Style/MutableConstant was updated to consider dstr nodes as unfrozen in Ruby 3.0 even when frozen_string_literal: true is specified, and requires .freeze on those strings. However, a multiline string is considered a dstr node but does not contain interpolation so would still be frozen.

# frozen_string_literal: true

CONST = 'foo' \
        'bar'

CONST.frozen? #=> true

Fixes #10134.

---

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]

Nice 👍