Bumping Minimum MacOS version to 10.13

We will bump the minimum MacOS version from 10.9 (released in Oct. 2013, end-of-life since Dec. 2016) to 10.13 (released Sept. 2017, end-of-life since Dec. 2020). The main reason we managed to support 10.9 this long at all, is that conda-forge is able to ship an up-to-date C++ standard library for OSX, libcxx, superseding the old one present in the MacOS SDK on the system (at least from the point-of-view of the respective conda environments).

However, several core packages in the ecosystem now require at least 10.13 (or will very soon), in a way that we cannot be circumvent. These packages include libcxx, starting with version 17.0. This change will not affect already published artifacts, but in the near future, all new builds for OSX will require at least 10.13. This constraint will be implemented through the __osx virtual package, but the details of how we will achieve this are still being worked out. Only conda versions 4.8.0 or newer have this virtual package. If you are using a system with MacOS older than 10.13 and are using conda older than 4.8.0, you will need to either upgrade conda to at least 4.8.0 or upgrade your system to at least MacOS 10.13.