Example about how to use the policy enforcer with Spring

[pedroigor]

• Example about how to integrate the PEP in Spring Boot/Security applications
• Removing app-authz-rest-employee because it shows the same thing as the new example
• Temporarily enabling GHA for Spring quickstarts as they require the repository using JDK 17

This change set requires https://github.com/keycloak/keycloak/pull/20003.