Set principal claim to preferred_username.

This clearly defines what the principal name is expected to be passed
as.

Change-Id: I4e56ee1dc79ac2c7ef8af0e382065e84e2a05fe0
diff --git a/src/main/resources/application.properties b/src/main/resources/application.properties
index 833aa45..5a4fcde 100644
--- a/src/main/resources/application.properties
+++ b/src/main/resources/application.properties
@@ -23,6 +23,7 @@
 quarkus.oidc.auth-server-url = https://login.benkard.de/auth/realms/master
 quarkus.oidc.client-id = mulkcms
 quarkus.oidc.application-type = web-app
+quarkus.oidc.token.principal-claim = preferred_username
 
 quarkus.security.users.file.enabled = false
 quarkus.security.users.embedded.enabled = false