Simplify MapStruct Subclass Mappings

[raffaelfoidl]

Description
Up until version MapStruct version 1.4.2, subclass (downcast) mappings had to be implemented manually using instanceof checks. This has been a longstanding missing feature. A corresponding PR (mapstruct/mapstruct#2512) has been merged in October 2021 and made it into the release of 1.5.0.Beta2 (https://github.com/mapstruct/mapstruct/releases/tag/1.5.0.Beta2).

Since witness strives to rely on stable dependencies only, 1.5.0.Final (https://github.com/mapstruct/mapstruct/releases/tag/1.5.0.Final) is the first acceptable version shipping with the desired type refinement feature.

Related Problem
Due to partly complicated inheritance hierarchies, mapping between DTO and entity objects needs special handling with MapStruct. This issue applies to SetLogMapper (createDtoToEntity, dtoToEntity, entityToDto) and ExerciseMapper (entitiesToDtos and, by extension, entityToDtoTypeAware`).

Acceptance Criteria

• MapStruct has been updated to the newest stable version (>= 1.5.0.Final), see also https://mvnrepository.com/artifact/org.mapstruct/mapstruct
• manually, hard-coded instanceof-guided subclass mapping have been eliminated
  • SetLogMapper
  • ExerciseMapper

Note: The existing test infrastructure should cover all the changes introduced by this task. Therefore, the required modifications should be able to act as mapper-internal implementation specifics. This means no new test cases are required by this issue.