west: Point back to main after release

After releasing v3.1.0, move back to main.

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

Author: carlescufi

west.yml

@@ -12,5 +12,5 @@ manifest:
   projects:
     - name: zephyr
       remote: zephyrproject-rtos
-      revision: v3.1.0
+      revision: main
       import: true