Add breaking change note

docs/reference/migration/migrate_8_16.asciidoc

@@ -16,5 +16,22 @@ coming::[8.16.0]
 [[breaking-changes-8.16]]
 === Breaking changes
 
-There are no breaking changes in {es} 8.16.
+The following changes in {es} 8.16 might affect your applications
+and prevent them from operating normally.
+Before upgrading to 8.16, review these changes and take the described steps
+to mitigate the impact.
 
+[discrete]
+[[breaking_816_locale_change]]
+==== JDK locale database change
+
+{es} 8.16 changes the version of the JDK that is included from version 22 to version 23. This changes
+the locale database that is used by Elasticsearch from the _COMPAT_ database to the _CLDR_ database.
+This can result in significant changes to custom textual date field formats,
+and calculations for custom week-date date fields.
+
+For more information see <<custom-date-format-locales,custom date formats>>.
+
+If you run {es} 8.16 on JDK version 22 or below, it will use the _COMPAT_ locale database
+to match the behavior of 8.15. However, please note that starting with {es} 9.0,
+{es} will use the _CLDR_ database regardless of JDK version it is run on.