[dotnet] Fix targeting packs for test targets

Author: nvborisenko

dotnet/test/common/BUILD.bazel

@@ -39,9 +39,9 @@ csharp_library(
             "**/*Tests.cs",
         ],
     ),
-    target_frameworks = ["netstandard2.0"],
+    target_frameworks = ["net7.0"],
     targeting_packs = [
-        framework("nuget", "NETStandard.Library"),
+        framework("nuget", "Microsoft.NETCore.App.Ref"),
     ],
     visibility = [
         "//dotnet/test:__subpackages__",
@@ -77,7 +77,7 @@ dotnet_nunit_test_suite(
     ],
     target_frameworks = ["net7.0"],
     targeting_packs = [
-        framework("nuget", "NETStandard.Library"),
+        framework("nuget", "Microsoft.NETCore.App.Ref"),
     ],
     deps = [
         ":fixtures",