chore(vue3): simplify infinite scroll component

What does this MR do?

This is a simple refactoring, which moves logic from updated() hook to nextTick This is required since in Vue3 this.$options are read-only, so storing any data there will not work

However, this change, I believe simplifies entire logic and makes component more readable

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.
• [N/A] If it changes a Pajamas-compliant component's look & feel, the MR has been reviewed by a UX designer.
• [N/A] If it changes GitLab UI's documentation guidelines, the MR has been reviewed by a Technical Writer.
• [N/A] 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.