[nrf toup][nrfconnect] Replace POSIX_MAX_FDS with ZVFS_OPEN_MAX

Replace deprecated Kconfig.

Signed-off-by: Adrian Gielniewski <adrian.gielniewski@nordicsemi.no>

Author: adigie

config/nrfconnect/chip-module/Kconfig.defaults

@@ -40,7 +40,7 @@ config ASSERT_NO_MSG_INFO
 config HW_STACK_PROTECTION
     default y
 
-config POSIX_MAX_FDS
+config ZVFS_OPEN_MAX
     default 16
 
 config MAIN_STACK_SIZE