doc: add missing deprecated blocks to cluster

I believe that these are required so that the respective APIs are
marked as deprecated in the table of contents.

PR-URL: https://github.com/nodejs/node/pull/47981
Reviewed-By: Mestery <mestery@protonmail.com>
Reviewed-By: Mohammed Keyvanzadeh <mohammadkeyvanzade94@gmail.com>
Reviewed-By: Deokjin Kim <deokjin81.kim@gmail.com>

doc/api/cluster.md

@@ -850,6 +850,8 @@ added: v0.8.1
 deprecated: v16.0.0
 -->
 
+> Stability: 0 - Deprecated
+
 Deprecated alias for [`cluster.isPrimary`][].
 
 ## `cluster.isPrimary`
@@ -959,6 +961,8 @@ changes:
     description: The `stdio` option is supported now.
 -->
 
+> Stability: 0 - Deprecated
+
 Deprecated alias for [`.setupPrimary()`][].
 
 ## `cluster.setupPrimary([settings])`