Update shared_configuration.stack.next to 9.3.0 (#2511)

* Update shared_configuration.stack.next to 9.3.0

Updating the Assembler config to start publishing the upcoming minor version (9.3.0) to our staging site as per the instructions [here](https://stunning-adventure-qrvr1k2.pages.github.io/releases/elastic-stack-v9/).

* Update version to 9.3

Author: alexandra5000

config/assembler.yml

@@ -48,7 +48,7 @@ environments:
 shared_configuration:
   stack: &stack
     current:  9.2
-    next: main
+    next: 9.3
     edge: main
 
   master: &master