This is the correct case defined in the source code: https://github.com/krakjoe/apcu/blob/1f98e34d93/apc_iterator.h#L29

This is an attempt to fix errors that a project started to report on PHPStan `v0.12.26`:

```
 ------ ------------------------------------------------------------------
  Line   src/State/StateStorage.php
 ------ ------------------------------------------------------------------
  6      Class APCUIterator referenced with incorrect case: APCuIterator.
  109    Class APCUIterator referenced with incorrect case: APCuIterator.
 ------ ------------------------------------------------------------------
```