bmc-state-manager: Fix error in GetUnit

obmc-standby.target was removed in meta-phosphor:
  https://gerrit.openbmc-project.xyz/c/openbmc/meta-phosphor/+/20665
This caused an error in a phosphor-bmc-state-manager's GetUnit call.
Moving over to multi-user.target instead fixes the error.

Resolves: https://github.com/openbmc/phosphor-state-manager/issues/6

Tested: Verified the error no longer pops up

Change-Id: I3f0546284091f63717882db5998b9faa34855d6f
Signed-off-by: Anthony Wilson <wilsonan@us.ibm.com>
diff --git a/bmc_state_manager.cpp b/bmc_state_manager.cpp
index 20efcf0..92d9ef8 100644
--- a/bmc_state_manager.cpp
+++ b/bmc_state_manager.cpp
@@ -17,7 +17,7 @@
 using namespace phosphor::logging;
 using sdbusplus::exception::SdBusError;
 
-constexpr auto obmcStandbyTarget = "obmc-standby.target";
+constexpr auto obmcStandbyTarget = "multi-user.target";
 constexpr auto signalDone = "done";
 constexpr auto activeState = "active";