Change design task for Writing and Free flows (#39189)

The Write and Free launchpad checklists used the `design_selected` task
in their launchpad definitions. This task is always marked as done, even
if a design isn't actually selected, which is perfectly possible for
both of these flows - even likely for the write flow, depending upon the
precise path.

Now they use the `design_completed` task, which explicitly checks
whether a design has been selected before being marked as completed.

projects/packages/jetpack-mu-wpcom/changelog/update-launchpad-design-done

@@ -0,0 +1,4 @@
+Significance: patch
+Type: changed
+
+Launchpad: Require a design is selected before marking the step as complete in both the Write and Free flows

projects/packages/jetpack-mu-wpcom/src/features/launchpad/launchpad.php

@@ -52,7 +52,7 @@ function wpcom_launchpad_get_task_list_definitions() {
 			'task_ids'            => array(
 				'plan_selected',
 				'setup_free',
-				'design_selected',
+				'design_completed',
 				'domain_upsell',
 				'first_post_published',
 				'design_edited',
@@ -120,7 +120,7 @@ function wpcom_launchpad_get_task_list_definitions() {
 			},
 			'task_ids'            => array(
 				'setup_write',
-				'design_selected',
+				'design_completed',
 				'plan_selected',
 				'first_post_published',
 				'site_launched',