This mainly means the keycloak config moves to a separate bundle

The core bundle is now available on packagist

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

This is now handled by the composer.json in .gitlab-ci if needed

While we directed the docker image to translate the URLs to https anyway
this can be confusing since git behaves differently in docker vs outside.

Also in the meantime we have made all repos public, so there is no auth needed
for the repos and the extra work required using https no longer applies.

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.