Composer.json: add link to security policy

This is a new feature available since Composer 2.6.0, which was released a few weeks ago.

When this key is added, it will also show a link to the security policy on Packagist.

Refs:
* https://github.com/composer/composer/releases/tag/2.6.0
* composer/composer#11271
* composer/packagist#1353

composer.json

@@ -39,7 +39,8 @@
 	"support": {
 		"issues": "https://github.com/Yoast/yoast-acf-analysis/issues",
 		"forum": "https://wordpress.org/support/plugin/acf-content-analysis-for-yoast-seo",
-		"source": "https://github.com/Yoast/yoast-acf-analysis"
+		"source": "https://github.com/Yoast/yoast-acf-analysis",
+		"security": "https://yoast.com/security-program/"
 	},
 	"require": {
 		"php": "^7.2.5 || ^8.0",