Some config keys changed as well since the last update

It's used by symfony, use API_ instead

While the starter bundle gives a nice example for how to integrate an API
it makes setting up a new server harder because the user usually already
has a bundle in mind that he/she wants to add.

So remove it and instead make it easier to add bundles by improving the docs

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..