Sample for spring-projects/spring-security#8375.

Reproduction steps:

1. Build this project with ./gradlew assemble (requires Java 13)
2. Deploy the generated build/libs/spring-security-logout-1.0.0-SNAPSHOT.war to your favorite container.
3. Navigate to where you deployed (say http://localhost:8080/)
4. The home page will tell you are NOT authenticated.
5. Navigate to the /login page.
6. Authenticate with user:user credentials.
7. The home page will now tell you DO are authenticated.
8. Navigate to the /logout page and confirm.
9. Go back to the home page: it says you are still authenticated.