Remove deprecated _execute() method from ContactForm

CakePHP 5.3 deprecates _execute() in favor of process().
The process() method was already in place, just remove the
deprecated _execute() stub to avoid deprecation warnings.

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

Co-Authored-By: Claude <noreply@anthropic.com>

Author: dereuromark

src/Form/ContactForm.php

@@ -8,7 +8,7 @@
 
 /**
  * A default ContactForm form fitting most apps.
- * Extend in your app to fill _execute() and to customize
+ * Extend in your app to fill process() and to customize
  *
  * @author Mark Scherer
  * @license MIT
@@ -45,16 +45,6 @@ protected function _buildSchema(Schema $schema): Schema {
 			->addField('body', ['type' => 'text']);
 	}
 
-	/**
-	 * @deprecated Use process() instead.
-	 * @param array $data
-	 * @return bool
-	 */
-	protected function _execute(array $data): bool {
-		// Overwrite in your extending class
-		return true;
-	}
-
 	/**
 	 * @param array $data
 	 * @return bool