Use Windows 10 instead of 7 in user agent string
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I tried testing https://my.panomoments.com/ in the latest nightly version of Servo but it wouldn't load because it requires Windows 8 or newer and Servo's user agent string says it's using Windows 7. This change should allow sites requiring a modern version of Windows to run. This could potentially cause issues for users running older versions of Windows however. A better long term solution would be to do #18194.
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