Code Quality: Replace `void` with proper return types in Administration PHPDoc annotations.

Replace `void` in union return types with `null` or remove it where the function always returns a value or dies, across 8 files in `wp-admin/includes`. Adds explicit `return null;` statements where functions previously fell through without a return value.

Additionally:
* Adds `@return never` for `media_send_to_editor()` and removes misleading `return` from its callers.
* Removes `void` from return types where the function always returns a value or exits: `write_post()`, `WP_Importer::set_blog()`, `WP_Importer::set_user()`.
* Replaces `mixed|void` with a specific array shape for `WP_Site_Health::perform_test()`.

Developed in https://github.com/WordPress/wordpress-develop/pull/11008

Follow-up to r62178, r62177, r61766, r61719.

Props apermo, xate, westonruter, mukesh27, desrosj.
Fixes #64702.


git-svn-id: https://develop.svn.wordpress.org/trunk@62179 602fd350-edb4-49c9-b593-d223f7449a82