circleci: bump to latest xcode, macOS 10.15
circleci: bump to latest xcode, macOS 10.15

macOS 10.15 is Catalina+ and matches the current Docker Desktop limits
https://docs.docker.com/desktop/install/mac-install/

Signed-off-by: David Scott <bfcdf3e6ca6cef45543bfbb57509c92aec9a39fb@recoil.org>

djs55/vpnkit,djs55/vpnkit,djs55/vpnkit