linux: always use xz for compressing modules

The change I merged too hastily in NixOS#302300 increases the size by
~30%. This could be improved upon, but in the meantime let's go back
to xz while keeping zstd-compressed firmware (only 4.4% larger) and
_support_ for zstd-compressed modules.

nixos/doc/manual/release-notes/rl-2405.section.md

@@ -53,7 +53,7 @@ Use `services.pipewire.extraConfig` or `services.pipewire.configPackages` for Pi
 - `system.etc.overlay.enable` option was added. If enabled, `/etc` is
   mounted via an overlayfs instead of being created by a custom perl script.
 
-- For each supporting version of the Linux kernel firmware blobs and kernel modules
+- For each supporting version of the Linux kernel firmware blobs
   are compressed with zstd. For firmware blobs this means an increase of 4.4% in size, however
   a significantly higher decompression speed.
 

pkgs/os-specific/linux/kernel/common-config.nix

@@ -951,8 +951,7 @@ let
       ZEROPLUS_FF        = yes;
 
       MODULE_COMPRESS      = whenOlder "5.13" yes;
-      MODULE_COMPRESS_XZ   = whenOlder "5.13" yes;
-      MODULE_COMPRESS_ZSTD = whenAtLeast "5.13" yes;
+      MODULE_COMPRESS_XZ   = yes;
 
       SYSVIPC            = yes;  # System-V IPC