OZG-4949 Fix PostfachITCase put with Mongock

src/test/java/de/ozgcloud/admin/settings/postfach/PostfachITCase.java

@@ -236,6 +236,7 @@ class PostfachITCase {
 
 		@BeforeEach
 		void init() {
+			mongoOperations.dropCollection(Settings.class);
 			id = mongoOperations.save(settingsWithPostfach).getId();
 		}