Test on PHP 8.5

[jnoordsij]

WHY

BEFORE - What was wrong? What was happening before this PR?

Tests were not run on the latest PHP RC version (8.5; see https://www.php.net/archive/2025.php#2025-09-25-3).

AFTER - What is happening after this PR?

Tests run on PHP 8.5, in preparation for the stable release, to allow for spotting any deprecations or other issues.

HOW

How did you achieve that, in technical terms?

??

Is it a breaking change?

How can we test the before & after?

No

If the PR has changes in multiple repos please provide the command to checkout all branches, eg.:

git checkout "dev-branch-name" &&
cd vendor/backpack/crud && git checkout crud-branch-name &&
cd ../pro && git checkout pro-branch-name &&
cd ../../..

(other repos might require similar changes)