Collapse: Use $.fn.children instead of `>` in selector

Fixes #15131.

js/collapse.js

@@ -46,7 +46,7 @@
     if (this.transitioning || this.$element.hasClass('in')) return
 
     var activesData
-    var actives = this.$parent && this.$parent.find('> .panel').children('.in, .collapsing')
+    var actives = this.$parent && this.$parent.children('.panel').children('.in, .collapsing')
 
     if (actives && actives.length) {
       activesData = actives.data('bs.collapse')