HOTFIX: Only drop the CVSSRating if it exists (#7180)

Author: loic-sharma

src/NuGetGallery/Migrations/201904292139301_RemoveVulnerabilityDataFromPackageDeprecationModel.cs

@@ -17,7 +17,7 @@ public override void Up()
             DropIndex("dbo.PackageDeprecationCves", new[] { "Cve_Key" });
             DropIndex("dbo.PackageDeprecationCwes", new[] { "PackageDeprecation_Key" });
             DropIndex("dbo.PackageDeprecationCwes", new[] { "Cwe_Key" });
-            Sql("ALTER TABLE [dbo].[PackageDeprecations] DROP CONSTRAINT [dbo.PackageDeprecations_CVSSRating];");
+            Sql("ALTER TABLE [dbo].[PackageDeprecations] DROP CONSTRAINT IF EXISTS [dbo.PackageDeprecations_CVSSRating];");
             DropColumn("dbo.PackageDeprecations", "CvssRating");
             DropTable("dbo.Cves");
             DropTable("dbo.Cwes");