Bump version of System.Runtime.CompilerServices.Unsafe to preview coming from maintenance-packages (#12313)

* Add dotnet-libraries feed to NuGet.config

* Bump version of System.Runtime.CompilerServices.Unsafe to preview coming from maintenance-packages

* Filter the dotnet-libraries package source to only consume System.Runtime.CompilerServices.Unsafe.

Author: carlossanlop

NuGet.config

@@ -3,6 +3,7 @@
   <packageSources>
     <clear />
     <add key="dotnet-eng" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet-eng/nuget/v3/index.json" />
+    <add key="dotnet-libraries" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet-libraries/nuget/v3/index.json" />
     <add key="dotnet-tools" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet-tools/nuget/v3/index.json" />
     <add key="dotnet8-transport" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet8-transport/nuget/v3/index.json" />
     <add key="dotnet9-transport" value="https://pkgs.dev.azure.com/dnceng/public/_packaging/dotnet9-transport/nuget/v3/index.json" />
@@ -21,6 +22,9 @@
       <package pattern="runtime.*" />
       <package pattern="system.*" />
     </packageSource>
+    <packageSource key="dotnet-libraries">
+      <package pattern="System.Runtime.CompilerServices.Unsafe" />
+    </packageSource>
     <packageSource key="dotnet-tools">
       <package pattern="microsoft.*" />
       <package pattern="MicroBuild.Core.Sentinel" />

eng/Versions.props

@@ -24,7 +24,7 @@
     <MicrosoftNETCoreRuntimeCoreCLRPackageVersion>5.0.0-preview.7.20320.5</MicrosoftNETCoreRuntimeCoreCLRPackageVersion>
     <MicrosoftNETCoreILDAsmPackageVersion>10.0.0-alpha.1.24514.25</MicrosoftNETCoreILDAsmPackageVersion>
     <SystemDiagnosticsPerformanceCounterPackageVersion>10.0.0-alpha.1.24514.25</SystemDiagnosticsPerformanceCounterPackageVersion>
-    <SystemRuntimeCompilerServicesUnsafePackageVersion>6.0.0</SystemRuntimeCompilerServicesUnsafePackageVersion>
+    <SystemRuntimeCompilerServicesUnsafePackageVersion>6.1.0-preview.1.24511.1</SystemRuntimeCompilerServicesUnsafePackageVersion>
     <SystemRuntimeSerializationFormattersPackageVersion>10.0.0-alpha.1.24514.25</SystemRuntimeSerializationFormattersPackageVersion>
     <SystemSecurityCryptographyPkcsPackageVersion>10.0.0-alpha.1.24514.25</SystemSecurityCryptographyPkcsPackageVersion>
     <SystemSecurityCryptographyProtectedDataPackageVersion>10.0.0-alpha.1.24514.25</SystemSecurityCryptographyProtectedDataPackageVersion>