fix(kotlin-spring): use correct Jackson 3 property path for WRITE_DATES_AS_TIMESTAMPS

When using Spring Boot 4 with Jackson 3, the property path for
WRITE_DATES_AS_TIMESTAMPS changed from:
  spring.jackson.serialization.WRITE_DATES_AS_TIMESTAMPS
to:
  spring.jackson.datatype.datetime.WRITE_DATES_AS_TIMESTAMPS

This fix adds conditional logic to the kotlin-spring generator template
to use the correct property path based on useSpringBoot4 flag, matching
the behavior already implemented in the JavaSpring generator.

Fixes binding error:
'Failed to bind properties under spring.jackson.serialization to
java.util.Map<tools.jackson.databind.SerializationFeature, java.lang.Boolean>:
No enum constant tools.jackson.databind.SerializationFeature.WRITE_DATES_AS_TIMESTAMPS'

Author: takkiraz

modules/openapi-generator/src/main/resources/kotlin-spring/libraries/spring-boot/application.mustache

@@ -3,8 +3,15 @@ spring:
     name: {{title}}
 
   jackson:
+{{#useSpringBoot4}}
+    datatype:
+      datetime:
+        WRITE_DATES_AS_TIMESTAMPS: false
+{{/useSpringBoot4}}
+{{^useSpringBoot4}}
     serialization:
       WRITE_DATES_AS_TIMESTAMPS: false
+{{/useSpringBoot4}}
 
 server:
   port: {{serverPort}}

samples/server/petstore/kotlin-springboot-4/src/main/resources/application.yaml

@@ -3,8 +3,9 @@ spring:
     name: openAPIPetstore
 
   jackson:
-    serialization:
-      WRITE_DATES_AS_TIMESTAMPS: false
+    datatype:
+      datetime:
+        WRITE_DATES_AS_TIMESTAMPS: false
 
 server:
   port: 8080