Drop --bs-accordion-btn-focus-border-color and deprecate $accordion-button-focus-border-color

[julien-deramond]

Description

As discussed in https://github.com/orgs/twbs/discussions/39559, accordion button's border width is set to 0 and is never changed afterwards. The focus state is handled by a box-shadow.

Based on these elements, this PR:

• Remove the border-color rule from the focused .accordion-button
• Remove useless --bs-accordion-btn-focus-border-color
• Deprecate unused $accordion-button-focus-border-color

This might be referenced in the blog post.

Type of changes

• Refactoring (non-breaking change)

Checklist

• I have read the contributing guidelines
• My code follows the code style of the project (using npm run lint)
• My change introduces changes to the documentation
• (N/A) I have updated the documentation accordingly
• (N/A) I have added tests to cover my changes
• All new and existing tests passed

Live previews

• https://deploy-preview-39560--twbs-bootstrap.netlify.app/docs/5.3/components/accordion/

Related issues

• https://github.com/orgs/twbs/discussions/39559