Use sw_vers -productVersion to discover the macOS version

[tas50]

We're currently using an overly complex method of detecting the platform version:

platform_version=`sw_vers | awk '/^ProductVersion:/ { print $2 }' | cut -d. -f1,2`

At least on 10.15 this can be done without the need for awk. We should figure out if older versions of macOS include this flag:

platform_version=`sw_vers -productVersion | cut -d. -f1,2`

[btm]

Both still work on macOS 11 (beta) on the DTK and on macOS 11 (beta) on my regular MBP.

releng@virtmac01 ~ % sw_vers -productVersion | cut -d. -f1,2
11.0

releng@virtmac01 ~ % sw_vers | awk '/^ProductVersion:/ { print $2 }' | cut -d. -f1,2
11.0