Fix broken build on .NET 9 SDK

- Turn off scanning for vulnerable dependencies in downstream packages
- Turn off warning when targeting NETSTANDARD 1.x

Author: bkoelman

Directory.Build.props

@@ -10,6 +10,8 @@
     <CodeAnalysisRuleSet>$(MSBuildThisFileDirectory)CodingGuidelines.ruleset</CodeAnalysisRuleSet>
     <RunSettingsFilePath>$(MSBuildThisFileDirectory)tests.runsettings</RunSettingsFilePath>
     <JsonApiDotNetCoreVersionPrefix>5.6.1</JsonApiDotNetCoreVersionPrefix>
+    <NuGetAuditMode>direct</NuGetAuditMode>
+    <NoWarn>$(NoWarn);NETSDK1215</NoWarn>
   </PropertyGroup>
 
   <PropertyGroup>