Explanation
x.500 is not a supported user authentication method for Universal Control Plane (UCP). x.500 is a series of standards for directory services that define how distributed directory information can be accessed and managed over a network. x.500 is not an external authentication backend that UCP supports. UCP supports LDAP, Active Directory, and SAML as external authentication backends. References:
https://docs.docker.com/ee/ucp/admin/configure/external-auth/, https://en.wikipedia.org/wiki/X.500

Explanation
Enabling mutual TLS between the Docker client and server is not the purpose of Docker Content Trust.
According to the official documentation, the purpose of Docker Content Trust is to verify the integrity and publisher of all data received from a registry over any channel.
References: https://docs.docker.com/engine/security/trust/content_trust/

Explanation
Uninstalling 'docker-ce' package before installing 'docker-ee' package does upgrade Docker Engine CE to Docker Engine EE. Docker Engine CE is the free and open source edition of Docker Engine, while Docker Engine EE is the enterprise-ready edition that includes additional features and support. To upgrade from CE to EE, you need to uninstall the 'docker-ce' package and its dependencies, and then install the 'docker-ee' package from the Docker repository. References:
https://docs.docker.com/engine/install/centos/#upgrade-docker-after-using-the-convenience-script,
https://docs.docker.com/engine/install/centos/#install-using-the-repository