fix: Template contains unconditional ": Serializable" which creates wrong derive clause on data classes which have a parent as well as are serializable. (#19283)

samples/server/petstore/kotlin-springboot-bigdecimal-default/src/main/kotlin/org/openapitools/model/Apa.kt

@@ -42,7 +42,7 @@ data class Apa(
 
     @Schema(example = "null", description = "")
     @get:JsonProperty("gepa") val gepa: java.math.BigDecimal? = null
-) {
+    ) {
 
 }