Bluetooth: BAP: Update BAP broadcast source sample to new USB API

[Thalley]

Is your enhancement proposal related to a problem? Please describe.
The USB API used by the sample is being removed, and replaced with a better version.

Describe the solution you'd like
The sample should use the new USB audio API: https://docs.zephyrproject.org/latest/doxygen/html/group__uac2__device.html

Describe alternatives you've considered
N/A

Additional context
https://github.com/zephyrproject-rtos/zephyr/tree/main/samples/subsys/usb/uac2_explicit_feedback and https://github.com/zephyrproject-rtos/zephyr/tree/main/samples/subsys/usb/uac2_implicit_feedback can be used as references, as they use the new API