docs(sbb-checkbox-group): set props instead of attributes in story

[DavideMininni-Fincons]

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• I have read the Contributing Guidelines for this project.
• I agree to follow the Code of Conduct that this project adheres to.
• I have searched the pull request tracker for a Pull Request (PR) that matches the one I want to submit, without success.

Issue

This PR Closes #2499

Pull request checklist

Please check if your PR fulfills the following requirements:

• Tests for the changes have been added (for bug fixes / features)
• Docs have been reviewed and added / updated if needed (for bug fixes / features)

See Review Guidelines for more information what is checked during review process.

Changes

Changes in this pull request:

• callbacks on change now set 'checked' and 'indeterminate' props instead of attributes

Browsers

I tested the build on the following browsers:

• Firefox Desktop
• Chrome Desktop
• Edge Desktop
• Safari Desktop
• Chrome Mobile
• Safari Mobile

Screen readers

I tested the build on the following browsers:

• JAWS Firefox Desktop
• JAWS Chrome Desktop
• NVDA Firefox Desktop
• NVDA Chrome Desktop
• VoiceOver Safari Desktop
• VoiceOver Chrome Desktop
• VoiceOver Safari Mobile
• Android Accessibility Suite Chrome Mobile

Pull request type

Please check the type of change your PR introduces:

• Bugfix
• Feature
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no api changes)
• Build related changes
• Documentation content changes
• Other (please describe):

Does this introduce a breaking change?

• Yes
• No

Other information

[codecov-commenter]

Codecov Report

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Additional details and impacted files

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  Lines           ?    24921           
  Branches        ?     2029           
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  Hits            ?    23225           
  Misses          ?     1667           
  Partials        ?       29           

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[TomMenga]

LGTM 👍

Ps. I changed the title to docs(..):... because fix is a releasable unit and this change should not be part of the release changelog.
If you want to know more

[jeripeierSBB]

LGTM Thanks