[12.x] Require PHP 8.5

Problem/Motivation

Once we support PHP 8.5 fully and the 12.x branch is open, we can require PHP 8.5 for Drupal 12.0.0.

This will mean dropping testing on all PHP versions prior to PHP 8.5, so should not necessarily be a priority for early in the 12.x development cycle when we're still backporting most changes to 11.3/4.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet