mb_convert_case($str, MB_CASE_TITLE, "UTF-8"); doesn't convert the Greek last letter to ς but to σ

### Description

The following code:

```php
<?php
echo mb_convert_case("ΚΑΛΗΣΠΕΡΑ ΣΑΣ", MB_CASE_TITLE, "UTF-8"); 
```

Resulted in this output:
```
Καλησπερα Σασ
```

But I expected this output instead:
```
Καλησπερα Σας
```


### PHP Version

7.4.3

### Operating System

ubuntu 20.4