Make the Security Vulnerabilities Application not bundled by default

Related to https://extensions.xwiki.org/xwiki/bin/view/Extension/Extension/Security/UI/

Following this forum discussion https://forum.xwiki.org/t/make-the-security-scan-dashboard-deactivated-by-default/13215

We agreed to:

• stop bundling the extension security extension by default
• advertise it as experimental in the extension documentation and in the extension UI