Repurpose pulp_last_updated to failed_at (on-demand stream)

As we can't backport the RemoteArtifact.failed_at field, we are using
the pulp_last_updated for the same purpose. We assume this field is
not actively used in any part of the system.

pulpcore/content/handler.py

@@ -827,7 +827,7 @@ async def _stream_content_artifact(self, request, response, content_artifact):
         remote_artifacts = (
             content_artifact.remoteartifact_set.select_related("remote")
             .order_by_acs()
-            .exclude(failed_at__gte=timezone.now() - timedelta(seconds=protection_time))
+            .exclude(pulp_last_updated__gte=timezone.now() - timedelta(seconds=protection_time))
         )
         async for remote_artifact in remote_artifacts:
             try:
@@ -1126,7 +1126,7 @@ async def finalize():
         try:
             download_result = await downloader.run()
         except DigestValidationError:
-            remote_artifact.failed_at = timezone.now()
+            remote_artifact.pulp_last_updated = timezone.now()
             await remote_artifact.asave()
             await downloader.session.close()
             close_tcp_connection(request.transport._sock)