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

[Jonathan Gibbons]

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

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

Changes: https://git.openjdk.org/jtreg/pull/114/files
 Webrev: https://webrevs.openjdk.org/?repo=jtreg&pr=114&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903294
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jtreg/pull/114.diff
  Fetch: git fetch https://git.openjdk.org/jtreg pull/114/head:pull/114

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