Fix incorrect casing of Chr() and Ord() in class-pclzip.php

[Soean]

Replace Chr() and Ord() with their canonical lowercase forms chr() and ord().

This is flagged as a case-sensitivity violation by the upcoming PHP 8.6 RFC, which will emit E_DEPRECATED for function references that don't match their declared casing. Fixing it now keeps WordPress ahead of the deprecation.

Trac ticket: https://core.trac.wordpress.org/ticket/64897

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