usb: device_next: Remove experimental status

The device_next stack now supports almost all classes that were covered
by the legacy stack, and many controllers have been ported already.

See zephyrproject-rtos#42066 for more info.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>

subsys/usb/device_next/Kconfig

@@ -3,8 +3,7 @@
 # SPDX-License-Identifier: Apache-2.0
 
 menuconfig USB_DEVICE_STACK_NEXT
-	bool "New USB device stack [EXPERIMENTAL]"
-	select EXPERIMENTAL
+	bool "New USB device stack"
 	select UDC_DRIVER
 	imply HWINFO
 	help