guix: consolidate kernel headers to 5.15

Given no reason to use an older version of the kernel headers for the
non-RISCV linux builds, consolidate all Linux builds to 5.15.x.

Note that using older kernel headers isn't some sort of compatibility
"hack", and glibc explicitly recommends against doing so. See:
https://sourceware.org/glibc/wiki/FAQ#What_version_of_the_Linux_kernel_headers_should_be_used.3F.

contrib/guix/manifest.scm

@@ -130,6 +130,7 @@ chain for " target " development."))
       (license (package-license xgcc)))))
 
 (define base-gcc gcc-10)
+(define base-linux-kernel-headers linux-libre-headers-5.15)
 
 ;; Building glibc with stack smashing protector first landed in glibc 2.25, use
 ;; this function to disable for older glibcs
@@ -148,7 +149,7 @@ chain for " target " development."))
 (define* (make-bitcoin-cross-toolchain target
                                        #:key
                                        (base-gcc-for-libc gcc-7)
-                                       (base-kernel-headers linux-libre-headers-4.9)
+                                       (base-kernel-headers base-linux-kernel-headers)
                                        (base-libc (make-glibc-without-ssp glibc-2.24))
                                        (base-gcc (make-gcc-rpath-link base-gcc)))
   "Convenience wrapper around MAKE-CROSS-TOOLCHAIN with default values
@@ -604,7 +605,7 @@ inspecting signatures in Mach-O binaries.")
                  (cond ((string-contains target "riscv64-")
                         (make-bitcoin-cross-toolchain target
                                                       #:base-libc glibc-2.27/bitcoin-patched
-                                                      #:base-kernel-headers linux-libre-headers-4.19))
+                                                      #:base-kernel-headers base-linux-kernel-headers))
                        (else
                         (make-bitcoin-cross-toolchain target)))))
           ((string-contains target "darwin")