[12.x] Require PHP 8.5

Is PHP 8.5 the eventual minimum version of Drupal 12, or is this an interim step?

Answering myself based on the rule linked, PHP 8.5.0 is at alpha now with a beta expected August 14, 2025 and stable somewhere in November 2025: https://www.php.net/supported-versions -- later PHP versions will not yet be stable when Drupal 12 reaches beta, so it will be PHP 8.5.x for sure for Drupal 12.

Added the statement to the issue summary :)

Cross-referencing 📌 Discus adoption of #NoDiscard and PHP 8.5 Active where PHP 8.5 support is being added.