Refine ESQL docs handling of applies_to (elastic#125835) (elastic#125923)

This primarily splits the old preview:true warning from the newer applies_to approach. Since all of our current applies_to examples are actually just behaviour modifications of current functions, we do not use the official docs {applies_to} syntax. However there is code to make use of that in the case where we have an entirely new function which will appear in a new version.

Co-authored-by: Alexander Spies <[email protected]>

Author: craigtaverner

docs/reference/query-languages/esql/_snippets/functions/layout/categorize.md

@@ -666,10 +666,6 @@ FROM employees
 ## `LOOKUP JOIN` [esql-lookup-join]
 
 ::::{warning}
-```{applies_to}
-stack: preview 9.0, coming 9.1
-serverless: preview
-```
 This functionality is in technical preview and may be changed or removed in a future release. Elastic will work to fix any issues, but features in technical preview are not subject to the support SLA of official GA features.
 ::::
 
@@ -753,10 +749,6 @@ FROM Left
 ## `MV_EXPAND` [esql-mv_expand]
 
 ::::{warning}
-```{applies_to}
-stack: preview 9.0, coming 9.1
-serverless: preview
-```
 This functionality is in technical preview and may be changed or removed in a future release. Elastic will work to fix any issues, but features in technical preview are not subject to the support SLA of official GA features.
 ::::
 

docs/reference/query-languages/esql/_snippets/functions/layout/kql.md

@@ -17,4 +17,6 @@
     String version() default "";
 
     String description() default "";
+
+    boolean serverless() default true;
 }