Update VideoTrackList for new-style event pages

[Elchi3]

This PR updates VideoTrackList for new-style event pages. (see the template)

To deduplicate the pages, I used these commands:

yarn content delete Web/API/VideoTrackList/onaddtrack --redirect=/en-US/docs/Web/API/VideoTrackList/addtrack_event
yarn content delete Web/API/VideoTrackList/onremovetrack --redirect=/en-US/docs/Web/API/VideoTrackList/removetrack_event
yarn content delete Web/API/VideoTrackList/onchange --redirect=/en-US/docs/Web/API/VideoTrackList/change_event  

BCD is approved in mdn/browser-compat-data#13885

[github-actions]

Preview URLs

• https://pr12597.content.dev.mdn.mozit.cloud/en-US/docs/Web/API/VideoTrackList/addtrack_event
• https://pr12597.content.dev.mdn.mozit.cloud/en-US/docs/Web/API/VideoTrackList/change_event
• https://pr12597.content.dev.mdn.mozit.cloud/en-US/docs/Web/API/VideoTrackList/removetrack_event
• https://pr12597.content.dev.mdn.mozit.cloud/en-US/docs/Web/API/VideoTrackList

Flaws

Note! 3 documents with no flaws that don't need to be listed. 🎉

URL: /en-US/docs/Web/API/VideoTrackList
Title: VideoTrackList
on GitHub
Flaw count: 5

• macros:
  • wrong xref macro used (consider changing which macro you use)
  • wrong xref macro used (consider changing which macro you use)
• bad_bcd_links:
  • no explanation!
  • no explanation!
  • no explanation!

External URLs

URL: /en-US/docs/Web/API/VideoTrackList
Title: VideoTrackList
on GitHub

No new external URLs

---

URL: /en-US/docs/Web/API/VideoTrackList/addtrack_event
Title: VideoTrackList: addtrack event
on GitHub

No new external URLs

---

URL: /en-US/docs/Web/API/VideoTrackList/removetrack_event
Title: VideoTrackList: removetrack event
on GitHub

No new external URLs

---

URL: /en-US/docs/Web/API/VideoTrackList/change_event
Title: VideoTrackList: change event
on GitHub

No new external URLs

(this comment was updated 2022-02-03 12:48:10.828967)

[teoli2003]

Two open questions, else it looks good.

[teoli2003]

The "or null" to be removed twice and we are good to go.