Upgrade vendored Bootstrap as well

Also removed theming functionality, since it is not compatible with Bootstrap 5

app/assets/javascripts/rails_admin.js

@@ -7,7 +7,8 @@
 //=  require 'rails_admin/jquery-ui/widgets/autocomplete'
 //=  require 'rails_admin/moment-with-locales'
 //=  require 'rails_admin/bootstrap-datetimepicker'
-//=  require 'rails_admin/bootstrap/bootstrap'
+//=  require 'rails_admin/popper'
+//=  require 'rails_admin/bootstrap'
 
 //=  require 'rails_admin/filter-box'
 //=  require 'rails_admin/filtering-multiselect'

app/assets/stylesheets/rails_admin.scss

@@ -0,0 +1,32 @@
+@charset "UTF-8";
+
+/***  Variables  ***/
+
+@import "rails_admin/custom/variables";
+@import "rails_admin/styles/base/variables";
+
+/***  Mixins  ***/
+
+@import "rails_admin/styles/base/mixins";
+@import "rails_admin/custom/mixins";
+
+/***  Bootstrap ***/
+
+$font-size-base: 0.85rem !default;
+@import "rails_admin/bootstrap/bootstrap";
+
+/***  Libraries  ***/
+
+//@import "rails_admin/bootstrap-datetimepicker-build";
+@import "rails_admin/styles/filtering-select";
+@import "rails_admin/styles/filtering-multiselect";
+@import "rails_admin/styles/widgets";
+
+/*** Font-awesome ***/
+
+@import "rails_admin/font-awesome";
+
+/***  RailsAdmin Theming  ***/
+
+@import "rails_admin/styles/base/theming";
+@import "rails_admin/custom/theming";