Remove deprecated 'unload' event listener

[Adam Johnson]

Loading any ModelAdmin page in Chrome 117+ triggers this warning:

  Unload event listeners are deprecated and will be removed.
  1 source
  RelatedObjectLookups.js:203
  https://chromestatus.com/feature/5579556305502208

That’s here: ​https://github.com/django/django/blob/857b1048d53ebf5fc5581c110e85c212b81ca83a/django/contrib/admin/static/admin/js/admin/RelatedObjectLookups.js#L213-L215

The ​linked proposal points to ​pagehide as a more reliable alternative, which seems like it would be compatible with the goal of closing child popups.

[Natalia Bidart]

Thank you Adam for taking the time to create this ticket, and for linking adequate resources to do proper triage.

I'm accepting on the basis that this makes sense but I would like to see more research being done regarding what to use to replace unload. The linked MDN docs say:

The pagehide event is sent to a Window when the browser hides the current page in the process of presenting a different page from the session's history.
For example, when the user clicks the browser's Back button, the current page receives a pagehide event before the previous page is shown.

Is not immediately clear to me that this is a good replacement. From ​these Chrome docs we may need to evaluate visbilitychange as well.

[Pooja Vishwakarma]

PR link:
​https://github.com/django/django/pull/18977