To be honest, if I'm just playing around I usually end up writing a console application that sends device telemetry in a while loop. If I'm feeling fancy I'll add some delays or logic to it. Bake it into a Docker image and you could run it both locally or in Azure (Container Apps/Container Instance). Otherwise, there are a lot of MQTT device simulators to be found online that can do this for you.
Not what you asked, but if you're going to do IoT on Azure, you might be interested in Azure IoT Hub. It's not a full MQTT broker (but does support MQTT), but has a lot of extra functionality around message routing and device configuration. There is a nice Azure IoT Device Telemetry Simulator too.