symfony now requires framework.secret to be set
the person entity is gone from the core bundle, so no need to implement it.

The starter bundle gained an example configuration and this adds an example
config file to set it.

The main change is that the core bundle no longer contains tugraz
specifics, so we can remove the corresponding config.

This also pins api-platform/core since the new 2.6 release has various
breaking changes we haven't worked around yet.

Ideally we should fail gracefully when they are not defined..