Support Gradle 9

[wilkinsona]

It's likely that Gradle 9 will be released in time for Boot 3.5 in May 2025. Assuming that happens, we should add support for it in 3.5.

[wilkinsona]

It doesn't look like Gradle 9 is going to be released in time for 3.5.0-RC1. Fortunately, supporting Gradle 9 doesn't appear to be too hard so this may just be a documentation issue to declare support when the time comes.