Add a known issue for upgrading from 5.x to 6.3.0 (#31501)

This is due to #31482

Author: bleskes

docs/reference/release-notes/6.3.asciidoc

@@ -4,6 +4,8 @@
 [float]
 === Known issues
 
+Upgrades from any 5.x version will fail for indexes which are prepared using the <<indices-synced-flush, _synced flush API>>, or were automatically sync-flushed due to inactivity. If upgrading from those versions, please wait for 6.3.1 to be released.
+
 Clusters with a Gold or Platinum license that are upgrading to 6.3 will need to explicitly set
 `xpack.security.enabled: true` in `elasticsearch.yml` to upgrade successfully.
 If this value is not set, the cluster will be unable to form after upgrade.