lang/php81: Extend EXPIRATION_DATE

As per the new RFC[1] the end of life of all current active PHP
versions has been extended by almost 1 year. Reflect the same in our
EXPIRATION_DATE based on the site[2].

1. https://wiki.php.net/rfc/release_cycle_update
2. https://www.php.net/supported-versions.php

Reported by: andreas.elsner@pitcom.de