system.text.json: update azure devops tests

Update Azure DevOps API tests to use System.Text.Json rather than
Json.NET for JSON serialization.

src/shared/Microsoft.AzureRepos.Tests/AzureDevOpsApiTests.cs

@@ -4,10 +4,10 @@
 using System.Net;
 using System.Net.Http;
 using System.Net.Http.Headers;
+using System.Text.Json;
 using System.Threading.Tasks;
 using GitCredentialManager;
 using GitCredentialManager.Tests.Objects;
-using Newtonsoft.Json;
 using Xunit;
 
 namespace Microsoft.AzureRepos.Tests
@@ -411,9 +411,10 @@ private static void AssertBearerToken(HttpRequestMessage request, string bearerT
 
         private static HttpResponseMessage CreateLocationServiceResponse(Uri identityServiceUri)
         {
-            var json = JsonConvert.SerializeObject(
-                new Dictionary<string, object>{["location"] = identityServiceUri.AbsoluteUri}
-            );
+            var json = JsonSerializer.Serialize(new Dictionary<string, object>
+            {
+                ["location"] = identityServiceUri.AbsoluteUri
+            });
 
             return new HttpResponseMessage(HttpStatusCode.OK)
             {
@@ -423,7 +424,7 @@ private static HttpResponseMessage CreateLocationServiceResponse(Uri identitySer
 
         private static HttpResponseMessage CreateIdentityServiceResponse(string pat)
         {
-            var json = JsonConvert.SerializeObject(
+            var json = JsonSerializer.Serialize(
                 new Dictionary<string, object> {["token"] = pat}
             );
 
@@ -435,7 +436,7 @@ private static HttpResponseMessage CreateIdentityServiceResponse(string pat)
 
         private static HttpResponseMessage CreateIdentityServiceErrorResponse(string errorMessage)
         {
-            var json = JsonConvert.SerializeObject(
+            var json = JsonSerializer.Serialize(
                 new Dictionary<string, object> {["message"] = errorMessage}
             );