CSS: Navbar toggler icon use mask instead of background-image

[louismaximepiton]

Description

Use mask instead of background-image

Motivation & Context

Reduce bundle size and will ease the color mode theming since the svg has to be written only once.

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) Since it removes a Sass variable.

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-39109--twbs-bootstrap.netlify.app/docs/5.3/components/navbar#toggler

Related issues

Might close something in #37549.