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.

contrib/guix/manifest.scm

@@ -11,7 +11,7 @@
              (gnu packages gawk)
              (gnu packages gcc)
              ((gnu packages installers) #:select (nsis-x86_64))
-             ((gnu packages linux) #:select (linux-libre-headers-5.15 util-linux))
+             ((gnu packages linux) #:select (linux-libre-headers-6.1 util-linux))
              (gnu packages llvm)
              (gnu packages mingw)
              (gnu packages moreutils)
@@ -92,7 +92,7 @@ chain for " target " development."))
       (license (package-license xgcc)))))
 
 (define base-gcc gcc-10)
-(define base-linux-kernel-headers linux-libre-headers-5.15)
+(define base-linux-kernel-headers linux-libre-headers-6.1)
 
 (define* (make-bitcoin-cross-toolchain target
                                        #:key