Update .NET references to point to 6.0

[vdice]

I wasn't able to build hippo with my .NET 5.0.x version, so I assume use of .NET 6 as of https://github.com/deislabs/hippo/pull/287 is a hard requirement. Hence, these docs updates.

Otherwise, if building hippo w/ .NET < 6.0 is still possible, happy to close and/or add different doc updates.

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[ghost]

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