Change all the XyzOptions.SystemClock properties to be EditorBrowsable(Never)

[Eilon]

And update all their doc comments to clearly say the are "For testing purposes only." (Some already make similar comments but aren't as clear.)

[Eilon]

Also need to check if JwtBearerOptions.SystemClock is even needed.

[brentschmaltz]

@Eilon the comment suggest this is just used for testing.

[HaoK]

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