tests: net: conn_mgr_nsos: fix CONFIG_ETH_NATIVE_TAP=n

Update option name s/CONFIG_ETH_NATIVE_POSIX/CONFIG_ETH_NATIVE_TAP/, which
follows recent Zephyr change.

Fixes 78f8006 ("drivers/ethernet/eth_native_posix: Rename to
  eth_native_tap")
Signed-off-by: Marcin Niestroj <m.niestroj@emb.dev>

Author: mniestroj

tests/net/conn_mgr_nsos/prj.conf

@@ -10,7 +10,7 @@ CONFIG_ENTROPY_GENERATOR=y
 CONFIG_TEST_RANDOM_GENERATOR=y
 CONFIG_ZTEST=y
 
-CONFIG_ETH_NATIVE_POSIX=n
+CONFIG_ETH_NATIVE_TAP=n
 CONFIG_NET_DRIVERS=y
 CONFIG_NET_SOCKETS=y
 CONFIG_NET_SOCKETS_OFFLOAD=y