Update custom SCSS script with new elements of BS 5.2.0.

Author: uhafner

etc/bootstrap-custom-build.scss

@@ -19,6 +19,7 @@ $container-max-widths: (
 ) !default;
 
 @import "../node_modules/bootstrap/scss/variables";
+@import "../node_modules/bootstrap/scss/maps";
 @import "../node_modules/bootstrap/scss/mixins";
 @import "../node_modules/bootstrap/scss/utilities";
 
@@ -62,6 +63,8 @@ th {
 @import "../node_modules/bootstrap/scss/popover";
 @import "../node_modules/bootstrap/scss/carousel";
 @import "../node_modules/bootstrap/scss/spinners";
+@import "../node_modules/bootstrap/scss/offcanvas";
+@import "../node_modules/bootstrap/scss/placeholders";
 @import "../node_modules/bootstrap/scss/helpers";
 
 @import "../node_modules/bootstrap/scss/utilities/api";