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)
• [x] Refactoring (non-breaking change)
• [x] Breaking change (fix or feature that would change existing functionality) Since it removes a Sass variable.

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
• [x] I have added tests to cover my changes
• [x] 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.

[mdo]

I'm down to see this tried out here and elsewhere if it makes sense (close button as an example).