[TASK] Do not use to-be deprecated `unload` event

Google Chrome (and other browser vendors will probably follow)
deprecates the unload event with Chrome 117 [1] and encourages using
the Page Lifecycle API [2] instead. This patch replaces any usage of
the `unload` event with the `pagehide` event that is available for all
supported browsers [3].

[1] https://developer.chrome.com/en/blog/deprecating-unload/
[2] https://developer.chrome.com/blog/page-lifecycle-api/
[3] https://caniuse.com/mdn-api_window_pagehide_event

Resolves: #101662
Releases: main, 12.4, 11.5
Change-Id: I675b180e855d8fb1070a84f5ba865e9250b2c0aa
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/80560


Reviewed-by: Benjamin Franzke <ben@bnf.dev>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Benjamin Franzke <ben@bnf.dev>