Remove custom OffsetDateTime mappings in OpenAPI config and ensure timestamps are converted to UTC in ChatLog.

web-api/build.gradle.kts

@@ -45,9 +45,6 @@ tasks.named<org.openapitools.generator.gradle.plugin.tasks.GenerateTask>("openAp
         )
     )
 
-    typeMappings.set(mapOf("OffsetDateTime" to "Instant"))
-    importMappings.set(mapOf("OffsetDateTime" to "java.time.Instant"))
-
     generateApiTests.set(false)
     generateApiDocumentation.set(false)
     generateModelTests.set(false)