guix: switch to 6.1 kernel headers over 5.15
Summary:
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.
Backport of core#28786.
Depends on D17230.
Test Plan: Run the guix builds.
Reviewers: #bitcoin_abc, PiRK
Reviewed By: #bitcoin_abc, PiRK
Differential Revision: https://reviews.bitcoinabc.org/D17231