Previously, we tried to use the `Backend` concept for both data persistence and data integration. 
This led to complicated cross-dependencies, architectures and badly maintainable code. 

Now, we decided to split these two concepts and use the existing `Backend` `ObjectStore`s for just the data persistence.

Concretely, this commit does the following:

1. Remove the abstract `Backend` class (backends.py), as well as their concrete implementations (`CouchDBBackend`, `LocalFileBackend`), as they are no longer needed
2. Remove the `Referable.source` attribute, as it was only needed for the `Backend` class
3. Remove methods related to the `Referable.source` attribute, such as `Referable.update()` and `Referable.commit()` (base.py), as they were used together to perform the data integration.
4. Adapt other related codes such as tutorials and tests to reflect these changes
5. Adapt documentation to reflect these changes

Prepare Release v2.0.0

# Release Notes

Version 2.0.0 of the BaSyx-Python SDK comes with a major refactoring of the server and a renewed concept for data persistence.

Previously, the server code was split between the `sdk` and `server` packages (due to historic development of the code). 
Now all the code relevant just for the server is located in `server`, where it belongs. Since this means, some code that was previously in `sdk` is not there anymore, this is a breaking change and warranted the new major release.

> [!note]
> This release does not have any changes in implemented AAS specification versions. It is the preparotory release in order to get ready for the new versions of the specifications, as well as new features for the SDK, such as Registry and Discovery server.

> [!warning]
> Due to these major refactorings, there were some backward incompatible changes. Please check the documentation, if you encounter any issues.

# Changelog

**Notable:**
- Backward Incompatible: Refactor server functionality from `sdk` to `server` (See: #388)
- Backward Incompatible: Refactor `backend` concept for data persistence (See: #370)
- Backward Incompatible: Refactor server `start-up` options (See: #418)
- Remove support for Python 3.9 (as it is EoL) (See #433)

**Improvements:**
- Clarify documentation of running the server with Docker (See: #398)
- Document running the server without Docker (See: #403)
- Improve XML serialization (See: #406)
- Improve server reading of JSON and XML files (See: #408)
- Add more utility methods for `Referable` and `Key` handling (See: #410)

**Bugfixes:**
- Fix type issues found with a new version of `mypy` (See: #399)
- Fix parsing of `ConceptDescription`s in the server (See: #420)
- Update `pyecma376-2` and `lxml` dependencies (See: #419)