[BSv5] Drop bg-gradient-variant() mixin

[chalin]

Bootstrap 5 has dropped the bg-gradient-variant() mixin -- for details, see the corresponding entry in the migration guide.

To avoid this change, we've kept a custom version of the mixin over time. For example, see:

• Fix warning on bg-gradient-variant mixin #328

We should follow BSv5's lead and drop this mixin in favor of the the approach recommended by BS5 in their migration guide.

I'm marking this as a BSv5-migration followup so that progress can be made to address breaking changes.