Fix PostDate using browser timezone instead of WordPress site timezone

[s3rgiosan]

Description of the Change

PostDate was hardcoding the browser's timezone via Intl.DateTimeFormat().resolvedOptions().timeZone and passing it to
dateI18n(). This overrode the WordPress site's configured timezone, causing the editor to display incorrect timezone
abbreviations (e.g., WET instead of EST).

This PR removes the hardcoded browser timezone and adds an optional timezone prop to PostDate. When the prop is not
provided, dateI18n() correctly falls back to the WordPress site timezone from getSettings(), which is the expected
behavior. The prop can still be used to explicitly override the timezone when needed.

Closes #

How to test the Change

1. Set the WordPress site timezone to America/New_York (Settings → General) → the editor should show EST/EDT.
2. Pass a timezone="Europe/London" prop to PostDate → should override to GMT/BST.
3. Leave the timezone prop unset with a UTC site setting → should show UTC.

Changelog Entry

Fixed - PostDate now uses the WordPress site timezone instead of the browser timezone when formatting dates.

Credits

Props @s3rgiosan

Checklist:

• I agree to follow this project's Code of
  Conduct.
• I have updated the documentation accordingly.
• I have added Critical Flows, Test Cases, and/or End-to-End
  Tests to cover my change.