Drop duplicate --bs-body-bg-rgb declaration + reorder props

[julien-deramond]

Description

The main purpose of this PR is to drop a duplicate declaration of our --bs-body-bg-rgb.

bootstrap/scss/_root.scss

Line 40 in 79e006b

--#{$prefix}body-bg-rgb: #{to-rgb($body-bg)};

bootstrap/scss/_root.scss

Line 78 in 79e006b

--#{$prefix}body-bg-rgb: #{to-rgb($body-bg)};

While doing it, I grouped together --bs-body-* props within root-body-variables section.

Note: originally found out by @louismaximepiton. Thanks!

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)
• All new and existing tests passed