rename file

Author: KarlErickson

articles/spring-apps/enterprise/sunset-announcement.md renamed to articles/spring-apps/enterprise/retirement-announcement.md

@@ -6,12 +6,12 @@ items:
 - name: Overview
   expanded: true
   items:
+  - name: Retirement announcement
+    href: retirement-announcement.md
   - name: What is Azure Spring Apps?
     href: overview.md
   - name: What's new
     href: whats-new.md
-  - name: Sunset announcement
-    href: sunset-announcement.md    
   - name: Compare available plans
     href: plan-comparison.md
   - name: Manage costs