[clue]

This changeset updates the build environment from PHP 8.1 to PHP 8.5, which is also what our web hosting already runs. With Sculpin, Twig and Symfony updated in #160, the build now runs on PHP 8.5 without any deprecation notices.

It also removes the Composer platform setting that pinned our dependencies to PHP 8.1.8, a leftover from building the website on the web hosting itself before #127. The lock file keeps its current packages for now, so a future update may move a few Symfony components to 7.4.

Nothing should change on the website. The generated pages are identical before and after this change, and the integration and Playwright tests pass as before.

Builds on top of #160, #158 and #123
Refs #127

[Copilot]

🟢 Approval recommended

The narrow configuration changes consistently target PHP 8.5, preserve locked dependencies, and have no identified blocking issues.

No open issues 🎉

What changed in this PR

Updates the website’s Docker-based build environment to PHP 8.5, matching the hosting runtime and following the Sculpin dependency upgrade.

Changes:

• Switches the Docker PHP base image and minimum PHP requirement to 8.5.
• Removes the Composer PHP 8.1.8 platform override without changing locked package versions.
• Updates the documented PHP requirement.

File	Description
README.md	Documents PHP 8.5+ as the requirement.
Dockerfile	Uses PHP 8.5 for build and runtime stages.
composer.lock	Updates platform metadata and removes the override.
composer.json	Requires PHP 8.5+ and removes the platform pin.

_{Review effort: Balanced}