update: previous version backups not available after a major version upgrade (#976)

Signed-off-by: dorota <[email protected]>

Author: wojcik-dorota

docs/products/postgresql/howto/upgrade.md

@@ -71,8 +71,9 @@ To upgrade a PostgreSQL service:
     Upon clicking **Upgrade**:
     - The system applies the upgrade **immediately**.
     - The PostgreSQL instance can't be restored to the previous version.
-    - Backups cannot be used for procedures such as Point In Time Recovery since they were
-      created with an earlier version of PostgreSQL.
+    - Backups created with an earlier major version are no longer visible in the Aiven Console
+      and cannot be used for operations such as Point In Time Recovery (PiTR). You can
+      only use backups created after the major version upgrade for such purposes.
     :::
 
 1.  Select **Upgrade**.