[php-src] Issue #11141: `Could not open input file:` should be sent to stderr

Issue: https://github.com/php/php-src/issues/11141 Author: mvorisek ### Description Repro command: php non-existent.php Currently the output (Could not open input file: non-existent.php) is sent to stdout but it should be sent to stderr instead. I have discovered when using php for interprocess communication where the stdout might be consumed as regular data causing protocol failure hard to understand, as in my case, only the first 4 bytes were consumed (and from Coul it was hard to identify the problem). Simply said, stdout should never contain any other data then data printed by a valid script.