use @Slf4j (+logback) for logging instead of System.out/err.println (#165)

Co-authored-by: Michael Hoennig <michael@hoennig.de>
Reviewed-on: https://dev.hostsharing.net/hostsharing/hs.hsadmin.ng/pulls/165
Reviewed-by: Marc Sandlus <marc.sandlus@hostsharing.net>

src/main/resources/application.yml

@@ -68,8 +68,10 @@ metrics:
                 server:
                     requests: true
 
+# HOWTO set logging-levels for certain Java packages (trace, debug, info, warn, error)
 logging:
     level:
-        org:
-            springframework:
-                security: TRACE
+        org.springframework.security: info
+# HOWTO configure logging, e.g. logging to a separate file, see:
+#   https://docs.spring.io/spring-boot/reference/features/logging.html
+