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

@@ -19,7 +19,8 @@
     "support" : {
         "issues" : "https://github.com/PHPCSStandards/PHPCSUtils/issues",
         "source" : "https://github.com/PHPCSStandards/PHPCSUtils",
-        "docs"   : "https://phpcsutils.com/"
+        "docs"   : "https://phpcsutils.com/",
+        "security": "https://github.com/PHPCSStandards/PHPCSUtils/security/policy"
     },
     "require" : {
         "php" : ">=5.4",