bg-gradient-variant mixin has been deprecated

[chrira]

Waring from docker build

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 21:12 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 118:14 of themes/docsy/assets/scss/_boxes.scss
         from line 14:9 of stdin