Can’t see it right away. But here are a few hints.
a) your item should be defined:
Number Wohnzimmer_Temperatur "Wohnzimmer Temperatur [%.1f °C]" <temperature> {channel="mqtt:topic:mosquitto:Temperatur_WZ:temperature"}
I replaced “broker” with “mosquitto” since that is what id you gave to the bridge.
a) you’re defining the Bridge in lowercase (don’t know if that matters)
b) Are you receiving MQTT messages on the source topic?
mosquitto_sub -t zigbee2mqtt/Temperatur_WZ
I think fixing a would probably work.
Alse please post any errors from your Openhab logs.
Kind regards,
Bas