Fix Action-Scheduler PHP 8.4 Deprecations - Bump PHP minimum to 7.1

[Soean]

With PHP 8.4 I get a lot of deprecation messages like

PHP Deprecated: ActionScheduler_QueueCleaner::__construct(): Implicitly marking parameter $store as nullable is deprecated, the explicit nullable type must be used instead in /wp-content/plugins/stream/vendor/woocommerce/action-scheduler/classes/ActionScheduler_QueueCleaner.php on line 35

It was fixed in Action-Scheduler 3.9.0: https://github.com/woocommerce/action-scheduler/releases/tag/3.9.0
Action-Scheduler 3.9.0 requires PHP 7.1.

Changes

This pull request updates the minimum required PHP version for the project from 7.0 to 7.1 and bumps the required version of the woocommerce/action-scheduler dependency. These changes help ensure compatibility with newer PHP features and maintain dependency security.

PHP version requirement updates:

• Increased the minimum required PHP version from 7.0 to 7.1 in both composer.json and the WP_STREAM_MIN_PHP_VERSION constant in stream.php. [1] [2]
• Added the Requires PHP: 7.1 header to the plugin file stream.php for clearer documentation and compatibility checks.

Dependency updates:

• Updated the required version of woocommerce/action-scheduler from ^3.8 to ^3.9 in composer.json.

Checklist

• Project documentation has been updated to reflect the changes in this pull request, if applicable.
• I have tested the changes in the local development environment (see contributing.md).
• I have added phpunit tests.

Release Changelog

• Fix: Describe a bug fix included in this release.
• Update: Minimum required version of PHP is now 7.1.