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.

Includes adding other missing "support" keys.

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

composer.json

@@ -9,6 +9,11 @@
 			"email": "[email protected]"
 		}
 	],
+	"support": {
+		"issues": "https://github.com/Yoast/yoast-test-helper/issues",
+		"source": "https://github.com/Yoast/yoast-test-helper",
+		"security": "https://yoast.com/security-program/"
+	},
 	"require": {
 		"php": "^7.2.5 || ^8.0"
 	},