guix: switch to 6.1 kernel headers over 5.15

6.1 is the current longterm release: https://kernel.org/.

Note that using an older version of the kernel headers inside Guix, is not a "hack" for compatibility, and is explicitly recommended against by glibc:

https://sourceware.org/glibc/wiki/FAQ#What_version_of_the_Linux_kernel_headers_should_be_used.3F.

so using the latest version of the longterm headers seems appropriate.

The last time we changed this was when we consolidated all builds to 5.15, in #25006.