chore(06-02): download and include Action Scheduler library

- Downloaded Action Scheduler 3.9.3 from GitHub
- Placed in vendor/action-scheduler/ directory
- Included in main plugin file before other code for proper initialization
- Library will auto-initialize itself when required

vendor/action-scheduler/deprecated/ActionScheduler_Schedule_Deprecated.php

@@ -0,0 +1,29 @@
+<?php
+
+/**
+ * Class ActionScheduler_Abstract_Schedule
+ */
+abstract class ActionScheduler_Schedule_Deprecated implements ActionScheduler_Schedule {
+
+	/**
+	 * Get the date & time this schedule was created to run, or calculate when it should be run
+	 * after a given date & time.
+	 *
+	 * @param DateTime $after DateTime to calculate against.
+	 *
+	 * @return DateTime|null
+	 */
+	public function next( ?DateTime $after = null ) {
+		if ( empty( $after ) ) {
+			$return_value       = $this->get_date();
+			$replacement_method = 'get_date()';
+		} else {
+			$return_value       = $this->get_next( $after );
+			$replacement_method = 'get_next( $after )';
+		}
+
+		_deprecated_function( __METHOD__, '3.0.0', __CLASS__ . '::' . $replacement_method ); // phpcs:ignore WordPress.Security.EscapeOutput.OutputNotEscaped
+
+		return $return_value;
+	}
+}