conf/qemu: bump preferred kernel version to 6.1

6.1 has been soaking for a while now, so it is time to bump the
default qemu version to 6.1 and prepare to remove 5.19.

Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>