Add mhv_secure_messaging_ehr_crosswalk feature flag (#27537)

* Add mhv_secure_messaging_ehr_crosswalk feature flag

Adds Flipper feature flag for the EHR transition crosswalk banner that
shows VistA to Oracle Health triage group name mappings. This flag will
gate the frontend display of the crosswalk data.

Refs #138121

* Update features.yml

Author: batemapf

config/features.yml

@@ -1700,6 +1700,10 @@ features:
   mhv_secure_messaging_cerner_pilot_system_maintenance_banner:
     actor_type: user
     description: Enables/disables system maintenance banner for Secure Messaging Oracle Health Transition on VA.gov
+  mhv_secure_messaging_ehr_crosswalk:
+    actor_type: user
+    description: Enables/disables the EHR transition crosswalk banner showing VistA to Oracle Health triage group name mappings
+    enable_in_development: false
   mhv_secure_messaging_filter_accordion:
     actor_type: user
     description: Enables/disables Secure Messaging Filter Accordion re-design updates on VA.gov