In https://github.com/spring-projects/spring-boot/issues/44602 awaitility was upgraded from 4.2.2 to 4.3.0. From the changeset of 4.3.0 https://github.com/awaitility/awaitility/compare/awaitility-4.2.2...awaitility-4.3.0 the required Kotlin version was upgraded to 2 https://github.com/awaitility/awaitility/commit/2a9814b2045c35a6c7670c5d40a83cc7887c8844. Spring Boot 3.5 depends on Kotlin 1.9.x which breaks compilation of existing projects.