Update postman-smtp.php

[IanDelMar]

This PR introduces several updates to the postman-smtp.php file:

• Adopts the DocBlock plugin header style.
• Adds Requires at least and Requires PHP headers to ensure compatibility checks are clear and explicit.
• Standardises on post_smtp as the unique plugin slug (likely something that could and should be done in other files as well) to ensure post_smtp is used consistently and properly deprecates other slug versions.
• Implements version-based cache busting for registered scripts to ensure browsers correctly invalidate cached assets upon updates.
• Prints scripts in the footer.
• Updates the required PHP version in version_compare() checks to reflect that the plugin requires PHP 7.0. However, I believe WordPress performs compatibility checks natively, and if so, this check could be removed altogether.
• Adds DocBlocks and type hints to provide clearer documentation.
• Prevents overriding the WordPress global $requires_php_version to ensure no conflicts with WordPress' built-in version handling.

Please note that none of these changes has been tested.