DEV: Change default code_formatting_style to code-fences (#13135)

Code fences:
1. allow to have syntax highlighting by declaring a language
2. are easier to edit (the composer doesn't preserve indentation when inserting a new line)
3. are more popular

config/site_settings.yml

@@ -955,10 +955,10 @@ posting:
   code_formatting_style:
     client: true
     type: enum
-    default: "4-spaces-indent"
+    default: "code-fences"
     choices:
-      - 4-spaces-indent
       - code-fences
+      - 4-spaces-indent
   embed_any_origin: false
   embed_topics_list: false
   embed_set_canonical_url: false
@@ -1011,7 +1011,6 @@ posting:
   review_every_post:
     default: false
 
-
 email:
   email_time_window_mins:
     default: 10
@@ -1718,7 +1717,6 @@ spam:
     default: 1
     min: 1
 
-
 rate_limits:
   unique_posts_mins: 5
   rate_limit_create_topic: 15