Fixes an issue that happens with OpenTelemetryMiddleware when a message is requeued with Context.requeue().

Context.requeue() uses self.broker.kick to kick a message to the broker which skips the Middleware pre-send and post-send invocations that kicker does here:

Messages that are requeued with the same context variables will raise an error on post_save's .detach() because they are now running in a different async context.

The message's lifecycle on OpenTelemetryMiddleware will be:

• pre_send -> message.labels are injected with context here
• post_send
• pre_execute -> message.labels are extracted here and re-used
• post_execute
• requeue() happens -> skipping pre_send and post_send, therefore not renewing the context.
• pre_execute -> context inferred from message.labels again
• post_execute
• post_save -> Detaching the stale context here. Raises ValueError: Token was created in a different context

This change gets rid of post_save on OpenTelemetryMiddleware to detach the context on post_execute instead. Since, we are not actually doing anything database save related on the current post_save method, it seems fair to move everything under post_execute as this is where the task execution actually ends.