Refactor properties handling

The current properties handling in `edu.kit.scc.dem.wapsrv.app.WapServerConfig` seems to be overly complicated and may circumvent spring functionalities in regards to properties loading.

I am a bit scared to refactor because of the generation of the application properties file on install - and because I do not have enough knowledge about spring.

Things to consider:
- properties are not defined in a default application properties. It seems to be difficult to extend or change properties without breaking anything.
- wap server specific properties do not have a prefix, which might hinder some of the usual spring properties handling. Fixing/Changing this would break backwards compatibility in regards to existing configuration files in deployment, so a migration would be necessary.
- There is a `updateConfigFromOldVersions` functionality which may rewrite existing properties files. I do not really get what it does / why it is necessary.

Start by reading edu.kit.scc.dem.wapsrv.app.WapServerConfig and tracing how properties are loaded and how updateConfigFromOldVersions rewrites existing files. Check the install-time generation of the application properties file and document the compatibility constraints before proposing a refactor. Done means a simpler Spring-compatible configuration path with existing deployment files preserved or migrated safely.

Written by the indexing model from the issue text.