ENH: Take into account `stdlib_version` for determining macOS SDK version in `.ci_support/` and cf-ci-setup

[h-vetinari]

With the introduction of {{ stdlib("c") }} and macosx_deployment_target, we have a new & better way to specify the required macosx deployment target. However, that means if people change to the new way, we need to take that into account when setting up the CI image (i.e. download_osx_sdk.sh in the ci-setup).

Additionally, we should warn about doubly specified deployment targets in cbc.yaml. My suggested proposal is:

• warn in smithy if MACOSX_DEPLOYMENT_TARGET is specified in cbc.yaml
• error in smithy if MACOSX_DEPLOYMENT_TARGET is in contradiction with a local c_stdlib_version on osx (not the one coming from the global pinning)
• keep writing MACOSX_DEPLOYMENT_TARGET to the .ci_support/*.yaml files, filled by whatever is in c_stdlib_version on osx
• that way, we don't need to touch the conda-forge-ci logic, which can keep working as-is for MACOSX_DEPLOYMENT_TARGET / MACOSX_SDK_VERSION.

Thoughts? @isuruf @mbargull @beckermr

[beckermr]

SGTM!

We probably want to have a linting errors for certain things (like the first bullet point) in addition to smithy warnings.

[isuruf]

Let's do the warning for a few releases and switch to an error.