samples: usb: make USB DFU next sample default

Move legacy USB DFU sample to legacy directory and make USB DFU next
sample default.

Signed-off-by: Johann Fischer <[email protected]>

Author: jfischer-no

doc/connectivity/usb/device/usb_device.rst

@@ -212,7 +212,7 @@ DFU
 USB DFU class implementation is tightly coupled to :ref:`dfu` and :ref:`mcuboot_api`.
 This means that the target platform must support the :ref:`flash_img_api` API.
 
-See :zephyr:code-sample:`usb-dfu` sample for reference.
+See :zephyr:code-sample:`legacy-usb-dfu` sample for reference.
 
 USB Human Interface Devices (HID) support
 =========================================

doc/releases/release-notes-4.1.rst

@@ -942,7 +942,7 @@ New Samples
 * :zephyr:code-sample:`bluetooth_ccp_call_control_client`
 * :zephyr:code-sample:`bluetooth_ccp_call_control_server`
 * :zephyr:code-sample:`coresight_stm_sample`
-* :zephyr:code-sample:`dfu-next`
+* :zephyr:code-sample:`usb-dfu`
 * :zephyr:code-sample:`i2c-rtio-loopback`
 * :zephyr:code-sample:`lvgl-screen-transparency`
 * :zephyr:code-sample:`mctp_endpoint_sample`