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

@@ -26,7 +26,8 @@
     "support": {
         "issues": "https://github.com/PHPCSStandards/PHP_CodeSniffer/issues",
         "wiki": "https://github.com/PHPCSStandards/PHP_CodeSniffer/wiki",
-        "source": "https://github.com/PHPCSStandards/PHP_CodeSniffer"
+        "source": "https://github.com/PHPCSStandards/PHP_CodeSniffer",
+        "security": "https://github.com/PHPCSStandards/PHP_CodeSniffer/security/policy"
     },
     "extra": {
         "branch-alias": {