minor symfony#59090 Add CI job for PHP 8.5 (alexandre-daubois)

This PR was merged into the 6.4 branch.

Discussion
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Add CI job for PHP 8.5

| Q             | A
| ------------- | ---
| Branch?       | 6.4
| Bug fix?      | no
| New feature?  | no
| Deprecations? | no
| Issues        | -
| License       | MIT

Enables PHP 8.5 nightlies on the CI.

What motivated this early addition is the continuous effort of converting resources to actual classes. PHP 8.5 already embeds features like https://wiki.php.net/rfc/directory-opaque-object, which could already be handled in PRs such as symfony#59035.

Also we'll be able to fix future deprecations as soon as they appear during the next year.

Commits
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15d56bb Add an experimental CI job for PHP 8.5

Author: fabpot

.github/workflows/unit-tests.yml

@@ -33,6 +33,7 @@ jobs:
             mode: low-deps
           - php: '8.3'
           - php: '8.4'
+          - php: '8.5'
             #mode: experimental
       fail-fast: false