SpringBoot generateAntoraYml remains up-to-date when its attributes have changed

For example, the changes in https://github.com/spring-projects/spring-boot/commit/a405c4200607b3483169c93a09f61d8d5d3f3db4 added two new attributes (url-log4j2-docs and url-quartz-javadoc) but generateAntoraYml may not run as a result of such changes as it doesn't correctly consider the attributes as an input. This can cause the docs to fail to build due to missing attributes.