chore: upgrade to PHP 8.5

[anonymoususer72041]

This updates OpenCATS to require PHP 8.5 and aligns the Composer requirements, CI runtime, installer checks and Docker PHP image with that requirement.

The Composer dependencies are refreshed for PHP 8.5 and the mbstring extension is declared explicitly because it is required by the replacement for deprecated encoding helpers. The CI setup now also declares the PHP extensions needed by the application and test dependencies instead of relying on runner defaults.

The application code is updated to avoid deprecated PHP APIs by replacing strftime() usage, removing utf8_encode() calls, adding explicit casts where PHP 8.5 no longer accepts nullable values implicitly and updating legacy runtime patterns such as old implode() argument order, callable syntax and dynamic properties.

Legacy PHP compatibility code that is no longer needed with the new minimum PHP version is removed while preserving the session/login behavior required by the test suite.

The Docker image now uses php:8.5-fpm-alpine. The unrtf build step sets compatibility CFLAGS because unrtf 0.21.10 uses legacy C constructs that fail with newer Alpine/GCC defaults.

This also updates the custom BrowserKit client signature for the Symfony version pulled in by the refreshed dependencies and stabilizes the Behat company autocomplete wait so the tests continue to exercise the real UI suggestion element.

[anonymoususer72041]

Please merge #797 first

[ocjorge]

Thank you for this comprehensive PHP 8.5 upgrade. I closed my PR #809
since this covers the same ground more thoroughly and is already part
of the v1.0.0 roadmap.

I noticed two fixes from #809 that might not be covered here:

1. CareerPortal.php: Wrapping PHPMailer send() in try/catch with
   error_log() — without this, the Career Portal returns a fatal 500
   error when no mail server is configured, which breaks the candidate
   application flow entirely.

2. AJAXInterface.php: Guarding session_start() with
   session_status() === PHP_SESSION_NONE — prevents duplicate session
   warnings in AJAX calls.

Both were tested in production on PHP 8.4.21. Happy to open targeted PRs
for these specific fixes if they are not already covered in this PR or
in #797. Just let me know what would be most useful.