docs: add security advisories URL in SECURITY.md

Author: kenjis

SECURITY.md

@@ -20,6 +20,7 @@ This person will coordinate the fix and release process, involving the following
 - Confirm the problem and determine the affected versions.
 - Audit code to find any potential similar problems.
 - Prepare fixes for all releases still under maintenance. These fixes will be released as fast as possible.
+- Publish security advisories at https://github.com/codeigniter4/CodeIgniter4/security/advisories
 
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