Downgrade FSharp.Core to the lowest denominator from our main project: 6.0.1: achieve best backward compat testing

We want to remove the dependency in our tests on FsToolkit.ErrorHandling.TaskResult in PR #181 because it uses a high version of FSharp.Core. In other words, it disallows us to properly test TaskSeq with the lowest-denominator FSharp.Core it was compiled with, which we should do, to ensure stability in the widest range of forward-and-backward compatibility. The SmokeTests library, on the other hand, uses the highest RTM release of FSharp.Core and we don't really care about other deps there.

Author: abelbraaksma

src/FSharp.Control.TaskSeq.Test/FSharp.Control.TaskSeq.Test.fsproj

@@ -53,7 +53,7 @@
   <ItemGroup>
     <!-- align test project with minimal required version for TaskSeq -->
     <!-- we use 6.0.3 here and not 6.0.2 because TaskResult lib requires it-->
-    <PackageReference Update="FSharp.Core" Version="6.0.3" />
+    <PackageReference Update="FSharp.Core" Version="6.0.1" />
     <PackageReference Include="FsUnit.xUnit" Version="5.5.0" />
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.7.2" />
     <PackageReference Include="xunit" Version="2.5.3" />