chore: remove EOL netcoreapp3.1 test invocation from build script

The test project (SqlClient.Tests.fsproj) targets net8.0 and net462 only;
it has never targeted netcoreapp3.1. Running dotnet test against a framework
the project does not target is a no-op at best and a spurious warning at
worst. .NET Core 3.1 reached end-of-life in December 2022.

Remove the dead invocation so the build script only tests against the
two supported frameworks: net462 (for .NET Framework) and net8.0 (for
modern .NET).

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: 2 people

build/build.fs

@@ -285,7 +285,6 @@ Target.create "RunTests" (fun _ ->
         }
       try 
           DotNet.test (dnTestOptions "net462") testSlnPath
-          DotNet.test (dnTestOptions "netcoreapp3.1") testProjectPath
           DotNet.test (dnTestOptions "net8.0") testProjectPath
       with
       | ex ->