Remove F# core 6 ref in other projects

Author: gusty

src/FSharpPlus.TypeLevel/FSharpPlus.TypeLevel.fsproj

@@ -22,9 +22,6 @@
     <!--<Compile Include="Providers/Matrix.Provider.fs" />
     <Compile Include="Providers/TypeProviderAssembly.fs" />-->
   </ItemGroup>
-  <ItemGroup>
-    <PackageReference Update="FSharp.Core" Version="6.0.6" />
-  </ItemGroup>
   <ItemGroup>
     <ProjectReference Include="../FSharpPlus/FSharpPlus.fsproj" />
   </ItemGroup>

tests/FSharpPlus.Tests/FSharpPlus.Tests.fsproj

@@ -51,7 +51,6 @@
   </ItemGroup>
   <ItemGroup>
     <PackageReference Include="System.Runtime" Version="4.3.1" />
-    <PackageReference Update="FSharp.Core" Version="6.0.6" />
     <PackageReference Include="MathNet.Numerics.FSharp" Version="4.8.1" />
     <PackageReference Include="NUnit" Version="3.13.3" />
     <PackageReference Include="FsCheck" Version="2.16.5" />