change `cpp_options[["stan_threads"]]` to `cpp_options[["STAN_THREADS"]]`

[toddpocuca]

This issue was discussed in https://github.com/wlandau/instantiate/issues/30 and the solution recommended was to create an issue here to ask for capitalizing "stan_threads" in https://github.com/stan-dev/cmdstanr/blob/5f2a8c850890e54e6fccc8108e5a31cb08010ab8/R/cpp_opts.R#L149-L171.

I don't know if this would be possible or if it would also be allowed for both "stan_threads " and "STAN_THREADS" to point to the same setting internally, but this would fix an issue with getting instantiate to allow for pre-compiling with multi-threading allowed without generating a warning.

[jgabry]

Thanks for opening the issue. If I recall correctly, I think this particular issue is being handled as part of #1022 (is that right @katrinabrock?), which is an overhaul of how we handle the C++ options.