Update Directory.Packages.props (#41895)

Author: ViktorHofer

src/SourceBuild/content/Directory.Packages.props

@@ -1,6 +1,7 @@
 <Project>
 
   <PropertyGroup>
+    <ManagePackageVersionsCentrally Condition="'$(ManagePackageVersionsCentrally)' == ''">true</ManagePackageVersionsCentrally>
     <CentralPackageTransitivePinningEnabled>true</CentralPackageTransitivePinningEnabled>
     <!-- Using multiple feeds isn't supported by Maestro: https://github.com/dotnet/arcade/issues/14155. -->
     <NoWarn>$(NoWarn);NU1507</NoWarn>