Change the value of "x-client-os" header

[tongyuze]

eSTS expects we send x-client-os: Windows instead of win32 as there are no OS called win 32. win32 is not a recognized OS string in the current server-side platform detection logic.

[rayluo]

Stakeholders discussed offline and are evaluating alternative options. So, I am leaving a Request Changes review here for now, so that my other teammates will not merge this PR as-is.

[rayluo]

Summary of the offline discussion.

• We consider have service-side consume the client-side "win32" value which has existed since day one.
• Regardless, the user-agent value and x-client-os value, which is what CA relies on for platform detection in this case, is easily spoofable even without a proxy, which is why our public guidance advises against using platform information for Conditional Access controls.