Allow opt-in PHP 8.5 testing

Migrated issue

Reported by: andypost

Related to !431 (merged) !395 (merged)

Problem/Motivation

There's PHP 8.5 going to core #3512558: [CI] Run Unit tests on PHP 8.5 nightly and images are rebuild every Friday #3512443: Add a PHP 8.5 image for early testing

Proposed resolution

As Drupal core 11.2 released increase CORE_PHP_NEXT to 8.5
PS: contrib's early adopters can use https://hub.docker.com/r/drupalci/php-8.5-ubuntu-apache/tags

Remaining tasks

- wait 11.2 release
- create MR and promote

User interface changes

API changes

Data model changes

Related issue: Issue #3512443

Related issue: Issue #3512558

Related issue: Issue #3523596

Related issue: Issue #3557585