[DOCS] note breaking change from 7.8.1 in migration guide (#59642)

Co-authored-by: Adam Locke <adam.locke@elastic.co>
(cherry picked from commit b0c34020ed6a10fda8e6efa9af343bd283954ec5)

docs/reference/migration/migrate_7_8.asciidoc

@@ -9,6 +9,7 @@ your application to {es} 7.8.
 
 See also <<release-highlights>> and <<es-release-notes>>.
 
+* <<breaking_781_license_changes>>
 * <<breaking_78_aggregation_changes>>
 * <<breaking_78_mappings_changes>>
 * <<breaking_78_settings_changes>>
@@ -18,6 +19,26 @@ See also <<release-highlights>> and <<es-release-notes>>.
 
 //tag::notable-breaking-changes[]
 
+[discrete]
+[[breaking_781_license_changes]]
+=== License Information changes
+
+As of the `7.8.1` release
+
+.Displays Enterprise license as Platinum in /_xpack 
+[%collapsible]
+====
+*Details*
+The `GET /_license` endpoint displays Enterprise licenses as
+Platinum by default so that old clients (including Beats, Kibana and
+Logstash) know to interpret this new license type as if it were a
+Platinum license.
+
+This compatibility layer was not applied to the `GET /_xpack/`
+endpoint, which also displays a license type and mode. {es-pull}58217[#58217]
+====
+
+
 [discrete]
 [[breaking_78_aggregation_changes]]
 === Aggregation changes