Add necessary packages

Author: MaggieKimani1

src/Microsoft.OpenApi/Microsoft.OpenApi.csproj

@@ -1,6 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
     <PropertyGroup>
         <TargetFrameworks>netstandard2.0</TargetFrameworks>
+        <LangVersion>9.0</LangVersion>
         <GeneratePackageOnBuild>true</GeneratePackageOnBuild>
         <PackageIconUrl>http://go.microsoft.com/fwlink/?LinkID=288890</PackageIconUrl>
         <PackageProjectUrl>https://github.com/Microsoft/OpenAPI.NET</PackageProjectUrl>
@@ -36,7 +37,8 @@
     </PropertyGroup>
 
     <ItemGroup>
-        <PackageReference Include="Microsoft.SourceLink.GitHub" Version="1.0.0" PrivateAssets="All"/>
+        <PackageReference Include="Microsoft.AspNetCore.Mvc.Core" Version="2.2.5" />
+        <PackageReference Include="Microsoft.SourceLink.GitHub" Version="1.0.0" PrivateAssets="All" />
     </ItemGroup>
 
     <ItemGroup>