Don't update xunit.combinatorial to v2

Author: AArnott

.github/renovate.json

@@ -16,6 +16,10 @@
 			"matchPackageNames": ["xunit.runner.visualstudio"],
 			"allowedVersions": "<3.0"
 		},
+		{
+			"matchPackageNames": ["xunit.combinatorial"],
+			"allowedVersions": "<2.0"
+		},
 		{
 			"matchDatasources": ["dotnet-version", "docker"],
 			"matchDepNames": ["dotnet-sdk", "mcr.microsoft.com/dotnet/sdk"],