Fix OIDC token endpoint crash on Debian Buster/Bullseye

The return type of jwt.encode() changed from bytes in v1.x (Buster/Bullseye)
to str in v2.x (Bookworm). This let json.dumps crash on Buster und Bullseye
with "TypeError: Object of type bytes is not JSON serializable".

Flask v1.x (Buster/Bullseye) automatically uses simplejson.dumps instead of
json.dumps if it is installed. simplejson.dumps auto-converts bytes to str per
default. simplejson also happend to be installed in our CI images. This
prevented the bug from surfacing in CI tests. We removed simplejson from our
CI images in an external change.

Co-authored-by: Julian Rother <julian@cccv.de>