Spring Security XML OpenID sample should POST to logout

Describe the bug

Currently the XML OpenID sample uses GET /logout, which doesn't work since the DefaultLogoutPageGeneratingFilter is skipped for OpenID.

To Reproduce

Start up the OpenID sample, navigate to http://localhost:8080, specify an OpenID provider to log in with, and then click Logout. The result is a 404.

Expected behavior

Logout should take the user back to the entry point. Specifically, it should POST to the /logout endpoint and include the _csrf parameter.