[Backport 8.15] Mark connector.last_sync as private for now (#2751)

There is currently a bug when updating errors. When fixed (most likely for
8.16), this endpoint will be aded back to the specification.

(cherry picked from commit 0cf8326)

Co-authored-by: Quentin Pradet <[email protected]>

Author: github-actions[bot]

specification/connector/last_sync/ConnectorUpdateLastSyncRequest.ts

@@ -26,7 +26,7 @@ import { SyncStatus } from '../_types/Connector'
 /**
  * Updates last sync stats in the connector document
  * @rest_spec_name connector.last_sync
- * @availability stack since=8.12.0 stability=experimental
+ * @availability stack since=8.12.0 stability=experimental visibility=private
  * @availability serverless stability=experimental visibility=private
  * @doc_id connector-last-sync
  */