Composer.json: add link to security policy

This is a new feature available since Composer 2.6.0, which was released a little while ago.

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

The security policy itself has been added to the organisation `.github` repository and can be accessed via the `security/policy` link on each repo.

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

composer.json

@@ -23,7 +23,8 @@
   ],
   "support": {
     "issues": "https://github.com/PHPCSStandards/composer-installer/issues",
-    "source": "https://github.com/PHPCSStandards/composer-installer"
+    "source": "https://github.com/PHPCSStandards/composer-installer",
+    "security": "https://github.com/PHPCSStandards/composer-installer/security/policy"
   },
   "require": {
     "php": ">=5.4",