Add deprecation notice to dashboard import/export docs. (elastic#108826)

docs/api/dashboard-api.asciidoc

@@ -1,6 +1,8 @@
 [[dashboard-api]]
 == Import and export dashboard APIs
 
+deprecated::[7.15.0,Both of these APIs have been deprecated in favor of <<saved-objects-api-import>> and <<saved-objects-api-export>>.]
+
 Import and export dashboards with the corresponding saved objects, such as visualizations, saved
 searches, and index patterns.
 

docs/api/dashboard/export-dashboard.asciidoc

@@ -4,7 +4,9 @@
 <titleabbrev>Export dashboard</titleabbrev>
 ++++
 
-experimental[] Export dashboards and corresponding saved objects.
+deprecated::[7.15.0,Use <<saved-objects-api-export>> instead.]
+
+Export dashboards and corresponding saved objects.
 
 [[dashboard-api-export-request]]
 ==== Request

docs/api/dashboard/import-dashboard.asciidoc

@@ -4,7 +4,9 @@
 <titleabbrev>Import dashboard</titleabbrev>
 ++++
 
-experimental[] Import dashboards and corresponding saved objects.
+deprecated::[7.15.0,Use <<saved-objects-api-import>> instead.]
+
+Import dashboards and corresponding saved objects.
 
 [[dashboard-api-import-request]]
 ==== Request

docs/user/dashboard/dashboard.asciidoc

@@ -345,7 +345,7 @@ To share the dashboard with a larger audience, click *Share* in the toolbar. For
 [[import-dashboards]]
 == Export dashboards
 
-To automate {kib}, you can export dashboards as JSON using the <<dashboard-api,import and export dashboard APIs>>. It is important to export dashboards with all necessary references.
+To automate {kib}, you can export dashboards as NDJSON using the <<saved-objects-api-export, Export objects API>>. It is important to export dashboards with all necessary references.
 
 --
 include::tutorial-create-a-dashboard-of-lens-panels.asciidoc[]

src/plugins/legacy_export/README.md

@@ -1,3 +1,3 @@
-# `legacyExport` plugin
+# `legacyExport` plugin [deprecated]
 
 The `legacyExport` plugin adds support for the legacy saved objects export format.

src/plugins/legacy_export/server/plugin.ts

@@ -9,6 +9,7 @@
 import { Plugin, CoreSetup, PluginInitializerContext } from 'kibana/server';
 import { registerRoutes } from './routes';
 
+/** @deprecated */
 export class LegacyExportPlugin implements Plugin<{}, {}> {
   constructor(private readonly initContext: PluginInitializerContext) {}