Safely update menus in slide-menu service (glimmer compat)

[iStefo]

When updating to Ember 3.28, I got the following error involving ember-side-menu:

index.js:181 Uncaught Error: Assertion Failed: You attempted to update [object Object].default to "<(unknown):ember545>", but it is being tracked by a tracking context, such as a template, computed property, or observer. In order to make sure the context updates properly, you must invalidate the property when updating it. You can mark the property as `@tracked`, or use `@ember/object#set` to do this.
    at assert (index.js:181)
    at Object.set [as default] (index.js:899)
    at Class.create (side-menu.js:35)
    at Class._initMenu (side-menu.js:109)
    at Class.didInsertElement (side-menu.js:95)
    at Class.superWrapper [as didInsertElement] (index.js:445)
    at Class.trigger (core_view.js:68)
    at Class.superWrapper [as trigger] (index.js:445)
    at CurlyComponentManager.didCreate (index.js:1178)
    at TransactionImpl.commit (runtime.js:4815)

Using set resolves the issue.