Use async Task as async void is obsolete

Author: MaggieKimani1

test/Microsoft.OpenApi.Readers.Tests/OpenApiReaderTests/OpenApiStreamReaderTests.cs

@@ -2,6 +2,7 @@
 // Licensed under the MIT license.
 
 using System.IO;
+using System.Threading.Tasks;
 using Microsoft.OpenApi.Models;
 using Microsoft.OpenApi.Reader;
 using Xunit;
@@ -36,7 +37,7 @@ public void StreamShouldNotCloseIfLeaveStreamOpenSettingEqualsTrue()
         }
 
         [Fact]
-        public async void StreamShouldNotBeDisposedIfLeaveStreamOpenSettingIsTrue()
+        public async Task StreamShouldNotBeDisposedIfLeaveStreamOpenSettingIsTrue()
         {
             var memoryStream = new MemoryStream();
             using var fileStream = Resources.GetStream(Path.Combine(SampleFolderPath, "petStore.yaml"));

test/Microsoft.OpenApi.Tests/Models/OpenApiDocumentTests.cs

@@ -1342,7 +1342,7 @@ public void SerializeV2DocumentWithStyleAsNullDoesNotWriteOutStyleValue()
         [Theory]
         [InlineData(true)]
         [InlineData(false)]
-        public async void SerializeDocumentWithWebhooksAsV3JsonWorks(bool produceTerseOutput)
+        public async Task SerializeDocumentWithWebhooksAsV3JsonWorks(bool produceTerseOutput)
         {
             // Arrange
             var outputStringWriter = new StringWriter(CultureInfo.InvariantCulture);