change(xtensa): Deprecate ".../xtensa_api.h" include path

This commit deprecates the "freertos/xtensa_api.h" and "xtensa/xtensa_api.h"
include paths. Users should use "xtensa_api.h" instead.

- Replace legacy include paths
- Removed some unnecessary includes of "xtensa_api.h"
- Replaced some calls with "esp_cpu_..." equivalents
- Add warning to compatibility header

components/esp_system/test_apps/esp_system_unity_tests/main/test_backtrace.c

@@ -15,7 +15,7 @@
 
 #include "freertos/FreeRTOS.h"
 #include "freertos/task.h"
-#include "freertos/xtensa_api.h"
+#include "xtensa_api.h" // Replace with interrupt allocator API (IDF-3891)
 #include "esp_intr_alloc.h"
 #include "esp_rom_sys.h"
 #include "esp_rom_uart.h"