fix: override ManagePackageVersionsCentrally in test .csproj file

Author: wturk

modules/openapi-generator/src/main/resources/csharp/netcore_testproject.mustache

@@ -6,6 +6,7 @@
     <TargetFramework{{#multiTarget}}s{{/multiTarget}}>{{testTargetFramework}}</TargetFramework{{#multiTarget}}s{{/multiTarget}}>
     <IsPackable>false</IsPackable>{{#nrt}}
     <Nullable>{{#useGenericHost}}enable{{/useGenericHost}}{{^useGenericHost}}annotations{{/useGenericHost}}</Nullable>{{/nrt}}
+    <ManagePackageVersionsCentrally>false</ManagePackageVersionsCentrally>
   </PropertyGroup>
 
   <ItemGroup>