bluetooth: mesh: Use bt_get_name to get device name

By default if CONFIG_BT_MESH_PROXY_USE_DEVICE_NAME
or CONFIG_BT_MESH_PB_GATT_USE_DEVICE_NAME is enabled, the mesh stack
will add BT_DATA_NAME_COMPLETE AD Type along with the Mesh Proxy Service
or Mesh Provisioning Service advertisements accordingly.

When BT_LE_ADV_OPT_USE_NAME was present and
CONFIG_BT_DEVICE_NAME_DYNAMIC is enabled, the advertised name was
automatically updated by the host. This turned out to be a side-effect
rather than expected behavior and after #71700 this behavior waa
changed.

But customers use dynamic name feature.

This commit makes the mesh stack use bt_get_name to get the device name,
which returns runtime name if CONFIG_BT_DEVICE_NAME_DYNAMIC is enabled.

Signed-off-by: Pavel Vasilyev <pavel.vasilyev@nordicsemi.no>