MDL-85260 core: prevent background scrolling

After upgrading to Bootstrap 5 (Moodle v5), opening a modal no longer prevents the background content from scrolling.
This fix restores the intended behavior by mimicking Bootstrap 5's JavaScript logic that disables background scrolling
when a modal is open.

lib/amd/src/modal.js

@@ -898,6 +898,8 @@ export default class Modal {
             this.accessibilityShow();
             this.getModal().focus();
             $('body').addClass('modal-open');
+            const scrollbarWidth = window.innerWidth - document.documentElement.clientWidth;
+            $('body').css({overflow: "hidden", paddingRight: `${scrollbarWidth}px`});
             this.root.trigger(ModalEvents.shown, this);
             dispatchEvent('core/modal:shown', this, this.modal[0]);
 
@@ -931,6 +933,7 @@ export default class Modal {
                 // Hide the backdrop if we're the last open modal.
                 backdrop.hide();
                 $('body').removeClass('modal-open');
+                $('body').css({overflow: "", paddingRight: ""});
             }
 
             const currentIndex = parseInt(this.root.css('z-index'));