Address PR feedback and update C# version

Author: MaggieKimani1

src/Microsoft.OpenApi.Readers/Microsoft.OpenApi.Readers.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>

src/Microsoft.OpenApi.Readers/V2/OpenApiV2VersionService.cs

@@ -225,7 +225,7 @@ public T LoadElement<T>(ParseNode node) where T : IOpenApiElement
         /// <inheritdoc />
         public string GetReferenceScalarValues(MapNode mapNode, string scalarValue)
         {
-            throw new NotImplementedException();
+            throw new InvalidOperationException();
         }
     }
 }

src/Microsoft.OpenApi.Readers/V3/OpenApiV3VersionService.cs

@@ -167,7 +167,7 @@ public T LoadElement<T>(ParseNode node) where T : IOpenApiElement
         public string GetReferenceScalarValues(MapNode mapNode, string scalarValue)
         {
             var valueNode = mapNode.Where(x => x.Name.Equals(scalarValue))
-                .Select(x => x.Value as ValueNode).FirstOrDefault();
+                .Select(static x => x.Value).OfType<ValueNode>().FirstOrDefault();
 
             return valueNode.GetScalarValue();
         }