test(coverlet): bump version 8 (#7877)

Author: ArgoZhang

test/Directory.Build.props

@@ -15,7 +15,7 @@
       <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
       <PrivateAssets>all</PrivateAssets>
     </PackageReference>
-    <PackageReference Include="coverlet.collector" Version="6.*">
+    <PackageReference Include="coverlet.collector" Version="8.*">
       <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
       <PrivateAssets>all</PrivateAssets>
     </PackageReference>