Metadata API: Stop using a deprecated method

persist_temp_file() is deprecated, and seemingly not very useful.

Signed-off-by: Jussi Kukkonen <jkukkonen@google.com>

tuf/api/metadata.py

@@ -36,7 +36,6 @@
 
 from securesystemslib.signer import Signature, Signer
 from securesystemslib.storage import FilesystemBackend, StorageBackendInterface
-from securesystemslib.util import persist_temp_file
 
 # Expose payload classes via ``tuf.api.metadata`` to maintain the API,
 # even if they are unused in the local scope.
@@ -332,11 +331,14 @@ def to_file(
             StorageError: The file cannot be written.
         """
 
+        if storage_backend is None:
+            storage_backend = FilesystemBackend()
+
         bytes_data = self.to_bytes(serializer)
 
         with tempfile.TemporaryFile() as temp_file:
             temp_file.write(bytes_data)
-            persist_temp_file(temp_file, filename, storage_backend)
+            storage_backend.put(temp_file, filename)
 
     # Signatures.
     def sign(