Fix links to LTS versions for Windows in README.md

[SteveL-MSFT]

PR Summary

The links to the LTS versions for Windows was pointing to the stable version. Now that 7.1 is out and is NOT LTS, it's not pointing to 7.0.3 which is current LTS. Fix is to separate out the links for LTS and stable and reflect those correctly in the table.

PR Context

Fix #14045

PR Checklist

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• Make sure all .h, .cpp, .cs, .ps1 and .psm1 files have the correct copyright header
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[ghost]

🎉v7.2.0-preview.1 has been released which incorporates this pull request.:tada:

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