ABCL working with openjdk21

Greetings, Bear fans. It seems that ABCL [works well with JDK21][1] without needing any changes. [1]: https://github.com/armedbear/abcl/actions/runs/6576709219/job/17866716581?pr... -- "A screaming comes across the sky. It has happened before but there is nothing to compare to it now."