Build failure on NUnitLite

[SeanKilleen]

In the build for all branches including master, we currently see:

Error: /usr/share/dotnet/sdk/6.0.428/Sdks/Microsoft.NET.Sdk/targets/Microsoft.NET.TargetFrameworkInference.targets(144,5): error NETSDK1045: The current .NET SDK does not support targeting .NET 8.0. Either target .NET 6.0 or lower, or use a version of the .NET SDK that supports .NET 8.0. [/home/runner/work/docs/docs/docs/snippets/Snippets.NUnitLite/Snippets.NUnitLite.csproj]

[SeanKilleen]

OK, this one is easy -- we're explicitly installing 6.x instead of 8.x or 9.x 😅

Will get it fixed up shortly, along with some other dependencies in the build (node version, etc.)