springboot Broaden deprecation warning check in Gradle plugin's tests

The check for deprecation warnings that was added in https://github.com/spring-projects/spring-boot/commit/76f03a8cadad8a6aafb8d80a8c8856eed86e4e42 is narrower than intended. It needs to check when there's no version at which a deprecation warnings is expected not just when the current version is less than that version.