Update minimum WordPress version to 6.4

Updates the "Requires WP" header from 5.7 to 6.4 to align with current plugin standards and internal documentation.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: GaryJones

ad-code-manager.php

@@ -19,7 +19,7 @@
  * License URI:       http://www.gnu.org/licenses/gpl-2.0.txt
  * GitHub Plugin URI: https://github.com/Automattic/ad-code-manager/
  * Requires PHP:      7.4
- * Requires WP:       5.7
+ * Requires WP:       6.4
  */
 
 declare(strict_types=1);