Update shared_configuration.stack.next to 9.4

Set Assembler stack.next to the upcoming minor for staging validation
after feature freeze (Stack 9.4.0 GA).

Refs elastic/dev#3502

Made-with: Cursor

Author: shainaraskas

config/assembler.yml

@@ -57,7 +57,7 @@ environments:
 shared_configuration:
   stack: &stack
     current:  9.3
-    next: main
+    next: 9.4
     edge: main
 
   master: &master