Move NuGet.config to the root + add vuln info (#3123)

Ports PRs: #2443 and #3024 to release/5.1 branch

Author: cheenamalhotra

src/NuGet.config NuGet.config

@@ -4,4 +4,8 @@
     <clear />
     <add key="dotnet-public" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet-public/nuget/v3/index.json" />
   </packageSources>
+  <auditSources>
+    <clear />
+    <add key="nuget.org" value="https://api.nuget.org/v3/index.json" />
+  </auditSources>
 </configuration>

src/Directory.Build.props

@@ -61,12 +61,18 @@
     <NuGetCmd>$(NuGetRoot)nuget.exe</NuGetCmd>
     <!-- Respect environment variable for the .NET install directory if set; otherwise, use the current default location -->
     <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
+    <WarningsNotAsErrors>$(WarningsNotAsErrors);NU1901;NU1902;NU1903;NU1904;NU1905</WarningsNotAsErrors>
     <BuildSimulator Condition="'$(BuildSimulator)' != 'true'">false</BuildSimulator>
   </PropertyGroup>
   <PropertyGroup Condition="'$(BuildSimulator)' == 'true'">
     <DefineConstants>$(DefineConstants);ENCLAVE_SIMULATOR</DefineConstants>
   </PropertyGroup>
 
+  <!-- Audit Settings -->
+  <PropertyGroup>
+    <NuGetAuditMode>all</NuGetAuditMode>
+  </PropertyGroup>
+  
   <!-- Packaging  for source link-->
   <PropertyGroup>
     <DebugType>portable</DebugType>