Refactor to remove `app/assets/javascripts/whats_new/index.js`

The following discussion from !138902 (merged) should be addressed:

• @mrincon started a discussion: (+4 comments)

  Question: Do we need the element #js-whats-new-app to be in the rendered in the page in the first place?

  We use display_whats_new? to decide if we render this in the page.html.haml, but we are also checking this.sidebarData.display_whats_new in this component.

  Could create a div or use <component> directly in this component that mounts this async?

See more at !138902 (comment 1694685765)