Fermat upgraded to Ubuntu 24.04

The benchmarking server fermat.caelum.ci.dev has been upgraded from Ubuntu 22.04 to 24.04, taking the kernel from 5.15 to 6.8. The trigger was a request from Thomas, who was trying to benchmark the effect of some io_uring flags that are not available on 5.15. The newer kernel covers every io_uring addition made since 5.15, so those benchmarks can now proceed.