[Fix #7241] Make Style/FrozenStringLiteralComment match only true & false

[tejasbubane]

Closes #7241

• 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]

I'm fine with the proposed fix. Probably we should also add some lint cop that checks for bad values in magic comments, but that's a separate topic.

[jonas054]

@tejasbubane Looks ready to merge, but there's a conflict. Can you fix it?

[tejasbubane]

@jonas054 Fixed.