Docs: Remove Bootstrap 4 link from footer

[coliff]

Description & Motivation & Context

Bootstrap 4 is EOL, so remove the link.

Type of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Refactoring (non-breaking change)
• [ ] Breaking change (fix or feature that would change existing functionality)

Checklist

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

Live previews

• https://deploy-preview-40924--twbs-bootstrap.netlify.app/

Related issues

Fixes: #40923

[louismaximepiton]

I'm fine with deleting this.

[mdo]

Thanks, but going to wait to do this until v6. We'll leave it in v5 for now.