fix: downgrade coverlet packages to 6.0.4 for net6.0 compatibility

coverlet.collector and coverlet.msbuild v8.0.1 only ship build assets
for net8.0, causing 'XPlat Code Coverage' data collector not found
errors when running tests on the net6.0 test project.

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

Author: gavinbarron

tests/Microsoft.Graph.DotnetCore.Core.Test/Microsoft.Graph.DotnetCore.Core.Test.csproj

@@ -25,11 +25,11 @@
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.13.0" />
     <PackageReference Include="Moq" Version="4.20.72" />
     <PackageReference Include="xunit" Version="2.9.3" />
-    <PackageReference Include="coverlet.collector" Version="8.0.1">
+    <PackageReference Include="coverlet.collector" Version="6.0.4">
       <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
         <PrivateAssets>all</PrivateAssets>
     </PackageReference>
-    <PackageReference Include="coverlet.msbuild" Version="8.0.1">
+    <PackageReference Include="coverlet.msbuild" Version="6.0.4">
       <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
       <PrivateAssets>all</PrivateAssets>
     </PackageReference>