Add PHP 8.5 support

[Copilot]

Composer reports version-constraint conflicts when installing yiisoft/mailer on PHP 8.5 because the package only declared compatibility up to PHP 8.4.

Changes

• composer.json — Extend require.php constraint from 8.1 - 8.4 → 8.1 - 8.5
• .github/workflows/build.yml — Add 8.5 to the PHPUnit test matrix

Original prompt

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This section details on the original issue you should resolve

<issue_title>PHP 8.5 support</issue_title>
<issue_description>### Proposed new feature or change

When installing or updating yiisoft/mailer on PHP 8.5, Composer reports version‑constraint conflicts. It appears that the package does not yet declare compatibility with PHP 8.5.</issue_description>

<agent_instructions>Fix PHP 8.5 compatibility, add it to github actions as well.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes PHP 8.5 support #121

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