Fix warning on bg-gradient-variant mixin

[hube12]

Hi,
Since i updated my modules i sadly have this error that spam my console:

WARNING: The `bg-gradient-variant` mixin has been deprecated as of v4.5.0. It will be removed entirely in v5.
         on line 8:5 of themes/docsy/assets/vendor/bootstrap/scss/mixins/_deprecate.scss, in mixin `deprecate`
         from line 22:12 of themes/docsy/assets/vendor/bootstrap/scss/mixins/_background-variant.scss, in mixin `bg-gradient-variant`
         from line 37:18 of themes/docsy/assets/scss/_boxes.scss, in mixin `box-variant`
         from line 126:14 of themes/docsy/assets/scss/_boxes.scss
         from line 14:9 of stdin

Would it be possible to switch to the new format which is :
The official @mixin for bg-variant and bg-gradient-variant in bootstrap 4.5.0 is now @mixin bg-variant($parent, $color, $ignore-warning: false) and @mixin bg-gradient-variant($parent, $color, $ignore-warning: false).

Also in 5.0 bootstrap is moving away from it The bg-gradient-variant() mixin is removed since the .bg-gradient class can now be used to add gradients to elements instead of the .bg-gradient-* classes. see https://github.com/twbs/bootstrap/blob/main/site/content/docs/5.0/migration.md

related deprecated pr: twbs/bootstrap#30594

Fix #327

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[hube12]

@googlebot I signed it!

[LisaFC]

Thanks, this is great!