Update documentation for Last-Modified header to avoid deprecated DATE_RFC7231 constant (PHP 8.5+)

[clue]

This changeset updates the documentation to avoid the deprecated DATE_RFC7231 constant (PHP 8.5+)

Builds on top of #52 and php/php-src#12989
Refs https://wiki.php.net/rfc/deprecations_php_8_5#deprecate_date_rfc7231_and_datetimeinterfacerfc7231 and https://www.php.net/manual/en/timezones.others.php

[Copilot]

Pull Request Overview

This PR updates documentation and code to avoid the deprecated DATE_RFC7231 constant in PHP 8.5+. The change replaces usage of the deprecated constant with manual date formatting using gmdate() to maintain HTTP-compliant date formatting.

• Replace DATE_RFC7231 constant with manual date formatting in documentation
• Update FilesystemHandler.php to use gmdate() with 'T' timezone identifier
• Expand documentation with clearer guidance on HTTP date formatting requirements

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/FilesystemHandler.php	Updates Last-Modified header generation to use gmdate() with timezone identifier instead of manual GMT append
docs/api/response.md	Replaces deprecated DATE_RFC7231 usage in examples and expands documentation with alternative formatting approaches

---

_{Tip: Customize your code reviews with copilot-instructions.md. Create the file or learn how to get started.}