ci: build separate macOS packages for macOS 10.13+ and macOS 10.10+

In order to support modern macOS features like Dark Mode, we need to use
Qt 5.15, which requires macOS 10.13.  However, we still want to support
macOS 10.10 as well, for which we need to use Qt 5.9.  Build separate
macOS packages for these use cases.

Fixes: #21606
Issue: #20825