[REF] website_slides: rename 'channel_management.xml' to 'slide_course_join.xml'

This commit simply renames a template file to better reflect its purpose and
also align with its JS widget file naming. It is a preparation commit for
pending changes.

Task 2045571
PR #41972

addons/website_slides/static/src/js/slides_course_join.js

@@ -9,7 +9,7 @@ var _t = core._t;
 
 var CourseJoinWidget = publicWidget.Widget.extend({
     template: 'slide.course.join',
-    xmlDependencies: ['/website_slides/static/src/xml/channel_management.xml'],
+    xmlDependencies: ['/website_slides/static/src/xml/slide_course_join.xml'],
     events: {
         'click .o_wslides_js_course_join_link': '_onClickJoin',
     },