RFR: CODETOOLS-7903294: Use `sw_vers` to get OS version on Mac

[Iris Clark]

On Thu, 8 Sep 2022 18:52:14 GMT, Jonathan Gibbons <jjg at openjdk.org> wrote:

> Please review a small change to use
>      `sw_vers -productVersion` 
> to obtain the macOS version, instead of using 
>     `defaults read loginwindow SystemVersionStampAsString`

Marked as reviewed by iris (Reviewer).

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PR: https://git.openjdk.org/jtreg/pull/114