Bluetooth: Mesh: Rename adv relay to adv simultaneous

Since notice that simultaneous advertising is not only used
by relay message, provision over pb-adv can also be used.
so it was changed to a more general name.

refs:https://github.com/zephyrproject-rtos/zephyr/pull/48903



Signed-off-by: Lingao Meng <menglingao@xiaomi.com>