boards native_sim: Deprecate CONFIG_NATIVE_SIM_NATIVE_POSIX_COMPAT

This option existed only to make the transition from native_posix to
native_sim easier. As native_posix is going to be removed in v4.2
we deprecate this option now, so it will also be removed.

We also switch this option to default to false already now.

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>
diff --git a/boards/native/native_sim/Kconfig b/boards/native/native_sim/Kconfig
index c2610e8..fc84add 100644
--- a/boards/native/native_sim/Kconfig
+++ b/boards/native/native_sim/Kconfig
@@ -18,12 +18,13 @@
 
 config NATIVE_SIM_NATIVE_POSIX_COMPAT
 	bool "Pretend to be a native_posix board"
-	default y
+	select DEPRECATED
 	help
 	  When this option is set the native_sim board will pretend to be
 	  a native_posix board from kconfig point of view, to allow using it directly with
 	  code which was meant for the native_posix board and checks for the macro
 	  CONFIG_BOARD_NATIVE_POSIX, or requires other kconfig options which depend on it.
+	  This option is deprecated and will be removed in Zephyr v4.3
 
 config NATIVE_SIM_SLOWDOWN_TO_REAL_TIME
 	bool "Slow down execution to real time"