Use .NET 8 target for docs generation, #928

[paulirwin]

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Uses the .NET 8 target for docs generation.

Related: #928

Description

The docs generation was using the net6.0 target as the latest available; as of #982 we now have the net8.0 target, so this updates the docfx json files to use that target instead.

[NightOwl888]

I have confirmed that the build works locally, and it looks like all of the files have been updated with net8.0. This looks good to merge.