Add automated test for "Replace Author when Deleting Users"

The manual test is here: Replace Author when Deleting Users

Existing automated coverage

• UsersGroupsRightsManagementIT#deleteUserWithScriptRights: U1 with Script right creates a page; admin disables then deletes U1; asserts the modal Script Rights / last author warning and that the user is deleted, without choosing a new author
• UsersGroupsRightsManagementIT#deleteUserWithProgrammingRights: same with Programming right
• UsersGroupsRightsManagementIT#createAndDeleteUser: a user can only be deleted once disabled
• ReplaceUserJobTest#updateAuthor: unit: the replace user job sets the new author on documents

Related: XWIKI-18504 (closed) added the test for the delete-user warning, but not for the replacement of the author.

Missing automated coverage

• In the delete confirmation modal of a user with Script right who last modified a page, select another user (or Admin) in the new author picker (#newAuthor; add a DeleteUserConfirmationModal method)
• Click Delete, open the page created by U1 and assert its last author is the selected user
• Optionally, select a user without Script right and assert the picker error message is shown

Suggested location: xwiki-platform-core/xwiki-platform-administration/xwiki-platform-administration-test/xwiki-platform-administration-test-docker/src/test/it/org/xwiki/administration/test/ui/UsersGroupsRightsManagementIT.java

Found by comparing the XWiki 18.4.4 manual test runs against the automated tests on master (a0055f10a1a).