Ignore warnings related to deprecated stdext::checked_array_iterator on MSVC (again)

[nickbianco]

Brief summary of changes

Update to the fix implemented in #3895.

Testing I've completed

Looking for feedback on...

CHANGELOG.md (choose one)

• no need to update because...internal build system fix.

---

This change is 

[adamkewley]

Looking into this a little, because I kept having issues with it downstream, the problem (iirc) is also be that the latest MSVC can't build OpenSim's dependencies/ either, due to the same warning. It wasn't caught in the previous PR because CI's caching the dependency build.

I ended up having to read into this particular deprecation more, and it turns out that MSVC has a very specific flag for disabling it:

• /D_SILENCE_STDEXT_ARR_ITERS_DEPRECATION_WARNING

Which is probably better to use than the /wd flag, because it is specific to this warning. In my downstream build, I'm abusing the fact that CXXFLAGS can be set as an environment variable at the top-level before building Opensim's dependencies:

https://github.com/ComputationalBiomechanicsLab/opensim-creator/blob/0.5.14/scripts/build_windows.py#L102

I wouldn't recommend deploying it this way (e.g. by telling users to set some environment variable) in opensim-core. The solution is probably going to be to use target_compile_definitions when configuring OpenSim's targets to add the flag, and then to separately update dependencies/CMakeLists.txt:197 by explicitly passing the flag via CMAKE_CXX_ARGS or similar to the spdlog build. I'm away for a few days so I can't handle this right now, but that's how I'd tackle it.

Or, even better, de-integrate spdlog+fmt and revert to something that uses snprintf etc., or wait a year for C++20 with <format> support to roll around to all target platforms :>

[nickbianco]

Thanks @adamkewley, hopefully that does the trick.

wait a year for C++20 with support to roll around to all target platforms :>

I think this is the move. fmt and spdlog are pretty heavily integrated via the logging system currently.

[adamkewley]

Seems to have done trick! Nice work @nickbianco !