users: Add user fails after distribution upgrade to trixie

Split from #2498 (closed).

ERROR plinth.actions Error running action users..create_user("ben", ****, "tester", ****): PermissionError("Invalid credentials")Action traceback:File "/freedombox/plinth/actions.py", line 398, in _privileged_call return_values = func(*arguments['args'], **arguments['kwargs'])File "/freedombox/plinth/modules/users/privileged.py", line 365, in create_user _validate_user(auth_user, auth_password)~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^File "/freedombox/plinth/modules/users/privileged.py", line 40, in _validate_user _validate_password(username, password)~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^File "/freedombox/plinth/modules/users/privileged.py", line 46, in _validate_passwordraise PermissionError('Invalid credentials') INFO plinth.actions # users..add_user_to_group("ben", "bit-torrent", "tester", ****)