Use input border SASS variables for check input

[mbrodala]

Description

Replace the CSS variables with $input-border-* SASS variables which are already used everywhere else. These in turn are based on CSS variables, so there should be no change of behavior.

Motivation & Context

This ensures consistency and simplifies input border adjustments for all kind of form fields.

Type of changes

• 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

• 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
• 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-41675--twbs-bootstrap.netlify.app/

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