chore: add and clean up deprecated namespaces

Author: MaggieKimani1

src/Microsoft.OpenApi/Services/OpenApiWorkspace.cs

@@ -4,6 +4,7 @@
 using System;
 using System.Collections.Generic;
 using System.IO;
+using System.Linq;
 
 namespace Microsoft.OpenApi
 {

test/Microsoft.OpenApi.Readers.Tests/V31Tests/RelativeReferenceTests.cs

@@ -3,11 +3,7 @@
 using System.Net.Http;
 using System.Text.Json.Nodes;
 using System.Threading.Tasks;
-using Microsoft.OpenApi.Models;
-using Microsoft.OpenApi.Models.Interfaces;
 using Microsoft.OpenApi.Reader;
-using Microsoft.OpenApi.Services;
-using Microsoft.OpenApi.Writers;
 using Xunit;
 
 namespace Microsoft.OpenApi.Readers.Tests.V31Tests