fix: use async method for crypto flush

Signed-off-by: Vincent Biret <vibiret@microsoft.com>

Author: baywet

src/Microsoft.OpenApi/Models/OpenApiDocument.cs

@@ -581,8 +581,11 @@ public async Task<string> GetHashCodeAsync(CancellationToken cancellationToken =
             using var streamWriter = new StreamWriter(cryptoStream);
 
             await WriteDocumentAsync(streamWriter, cancellationToken).ConfigureAwait(false);
-
+#if NET5_0_OR_GREATER
+            await cryptoStream.FlushFinalBlockAsync(cancellationToken).ConfigureAwait(false);
+#else
             cryptoStream.FlushFinalBlock();
+#endif
 
             var hash = sha.Hash;
 #endif