Improve gradients and add `.bg-gradient` helper class

Created by: MartijnCuppens

• Use a semitransparent gradient from light to dark which works on any background-color Restored
• Store the gradient as a custom property (--bs-gradient)
• Remove .bg-gradient-* variants in favour of .bg-gradient which works even when $enable-gradients are enabled
• Add gradients to navbar, active page links and badges when gradients are enabled

TODO:

• Open a PR to deprecate bg-gradient-variant() mixin in v4

Demo: https://deploy-preview-30583--twbs-bootstrap.netlify.app/docs/5.0/utilities/colors/#background-gradient