Document the deprecation of the 'auth.password' setting (#53822)

Author: danhermann

docs/reference/migration/migrate_7_7.asciidoc

@@ -51,6 +51,13 @@ will fail to start.
 The `order` config of authentication realms must be unique in version 8.0.0.
 If you configure more than one realm of any type with the same order, the node will fail to start.
 
+[discrete]
+[[deprecate-insecure-monitoring-password]]
+==== Deprecation of insecure monitoring password setting
+
+The `auth.password` setting for the monitoring HTTP exporter has been deprecated and will be
+removed in version 8.0.0. Please use the `auth.secure_password` setting instead.
+
 [discrete]
 [[breaking_77_search_changes]]
 === Search changes