nixos/stage-1: set zstd as the default compressor when supported

Co-authored-by: Atemu <atemu.main@gmail.com>

nixos/modules/system/boot/stage-1.nix

@@ -513,7 +513,12 @@ in
     };
 
     boot.initrd.compressor = mkOption {
-      default = "gzip";
+      default = (
+        if lib.versionAtLeast config.boot.kernelPackages.kernel.version "5.9"
+        then "zstd"
+        else "gzip"
+      );
+      defaultText = "zstd if the kernel supports it (5.9+), gzip if not.";
       type = types.unspecified; # We don't have a function type...
       description = ''
         The compressor to use on the initrd image. May be any of: