Upgrade QEMU version

We're currently version 7.2.13 of QEMU (the latest available in the stable Debian distribution), which is a couple of years old.

$ bazel run //labgrid/toolchain/qemu-system:resolved -- --version
QEMU emulator version 7.2.13 (Debian 1:7.2+dfsg-7+deb12u7)
Copyright (c) 2003-2022 Fabrice Bellard and the QEMU Project developers

It would be good to see what sort of benefits (e.g. performance improvements) we could get from using the latest version (9.2.0), which is available in the testing distribution.