chore(vue3): use scopedSlots instead of children for rendering

What does this MR do?

Refactor rendering of sorting item to rely on scopedSlots (slots are always present in scopedSlots in vue 2.6 and higher)

This allows not to use hacky context for testing

I've tested it locally and do not see any behavior changes, so treating this as a chore

This is forward-compatible change to support @vue/compat #1981 (closed)

Does this MR meet the acceptance criteria?

Conformity

• Code review guidelines.
• GitLab UI's contributing guidlines.
• If it changes a Pajamas-compliant component's look & feel, the MR has been reviewed by a UX designer.
• If it changes GitLab UI's documentation guidelines, the MR has been reviewed by a Technical Writer.
• If the MR changes a component's API, integration MR(s) have been opened in the following projects to ensure that the @gitlab/ui package can be upgraded quickly after the changes are released:
  • GitLab: mr_url
  • CustomersDot: mr_url
  • Status Page: mr_url
• Added the ~"component:*" label(s) if applicable.