Resolve inconsistency in Provider ID and sourcesystem_cd field usage in i2b2

There is currently an inconsistency in how we handle `provider_id` and `sourcesystem_cd` fields within our i2b2 instance, specifically regarding the identification of source systems and data providers.

Currently, different encodings are used in the `provider_id` field:
- `P21` for §21 KHEntgG data
- `@` for AKTIN data

These identifications should ideally be stored in `sourcesystem_cd`, which is designed for marking data sources. Due to data privacy restrictions preventing source storage in DWH, this isn't immediately problematic. However, this inconsistency could lead to confusion and misinterpretation, especially when integrating multiple data sources (e.g., Digital Rescue Chain)

Goal: Ensure clear separation between source system identification (`sourcesystem_cd`) and provider identification (`provider_id`) while maintaining compliance with data protection guidelines and i2b2 specifications.