chore: uses 31 serialization method

Author: baywet

src/Microsoft.OpenApi/Models/OpenApiDocument.cs

@@ -550,7 +550,7 @@ public async Task<string> GetHashCodeAsync(CancellationToken cancellationToken =
             async Task WriteDocumentAsync(TextWriter writer, CancellationToken token)
             {
                 var openApiJsonWriter = new OpenApiJsonWriter(writer, new() { Terse = true });
-                SerializeAsV3(openApiJsonWriter);
+                SerializeAsV31(openApiJsonWriter);
                 await openApiJsonWriter.FlushAsync(cancellationToken).ConfigureAwait(false);
             }
         }