OZG-5044: Simplify MapStruct Mapping from Postfach to

...in/java/de/ozgcloud/admin/settings/AlfaKonfiguration.java

...a/de/ozgcloud/admin/settings/AnwendungsKonfiguration.java

...ava/de/ozgcloud/admin/settings/DataRestConfiguration.java

src/main/java/de/ozgcloud/admin/settings/SettingMapper.java

...in/java/de/ozgcloud/admin/settings/SettingRepository.java

src/main/java/de/ozgcloud/admin/settings/SettingService.java

.../ozgcloud/admin/settings/VorgangManagerKonfiguration.java

...st/java/de/ozgcloud/admin/settings/SettingMapperTest.java

...t/java/de/ozgcloud/admin/settings/SettingServiceTest.java