[5.2.x] Fixed #36284, Refs #31170 -- Ensured related lookup popups are closed properly.

In the admin, when selecting related objects via the helpers defined in
`RelatedObjectLookups.js`, the `dismissRelatedLookupPopup` function was
attempting to access `window.relatedWindows`, which does not exist in
real execution, causing related lookup popups to remain open.

This change ensures that this code correctly accesses the module-local
`relatedWindows` by explicitly assigning it to `window.relatedWindows`.

Regression in 91bebf1adb.

Thanks Matthias Kestenholz for the report, the fix ideas, and testing.

Co-authored-by: Matthias Kestenholz <mk@feinheit.ch>

Backport of a245604277 from main.

js_tests/admin/RelatedObjectLookups.test.js

@@ -8,7 +8,6 @@ QUnit.module('admin.RelatedObjectLookups', {
             <input type="text" id="test_id" name="test" />
             <input type="text" id="many_test_id" name="many_test" class="vManyToManyRawIdAdminField" />
         `);
-        window.relatedWindows = window.relatedWindows || [];
     }
 });