[nrf fromtree] net: openthread: enable default software tx security in 1.3

Make sure MAC software transmission security is enabled by default
for Thread 1.3 builds as well.

Signed-off-by: Eduardo Montoya <[email protected]>
(cherry picked from commit 848fd44)

Author: edmont

subsys/net/l2/openthread/Kconfig.thread

@@ -105,7 +105,7 @@ config OPENTHREAD_PLATFORM_CSL_UNCERT
 
 config OPENTHREAD_MAC_SOFTWARE_TX_SECURITY_ENABLE
 	bool "Software transmission security logic"
-	default y if OPENTHREAD_THREAD_VERSION_1_2
+	default y if !OPENTHREAD_THREAD_VERSION_1_1
 
 config OPENTHREAD_MLE_INFORM_PREVIOUS_PARENT_ON_REATTACH
 	bool "Inform previous parent on reattach"