chore: Update docfx to 2.59.2
chore: Update docfx to 2.59.2

I suspect that installing .NET 6 on the CI machine caused the
previous version of docfx to fail. (It's always been somewhat picky
about SDK versions.)

googleapis/google-api-dotnet-client,googleapis/google-api-dotnet-client,googleapis/google-api-dotnet-client