default Linux kernel for RPI4 insecure

This needs updating for the recent security issues, e.g. copyfail - CVE-2026-31431 dirtyfrag - CVE-2026-43284, CVE-2026-43500 ssh-keysign-pwn - CVE-2026-46333 https://github.com/NixOS/nixos-hardware/blame/c97bc4d15bd3473dd095e8e8ba57330ab1943a77/raspberry-pi/common/kernel.nix#L28