linuxPackages: 4.19 -> 5.4

It's a longterm version that has been out for quite some time:
5.4.15 and 5.5 are current.  I've been using it, so far I'm not aware
of any issues with it.

Feature freeze for the next NixOS release is in two weeks,
so now seems to be high time to decide the default kernel version.
https://discourse.nixos.org/t/nixos-20-03-feature-freeze/5655

pkgs/top-level/all-packages.nix

@@ -16659,7 +16659,7 @@ in
   });
 
   # The current default kernel / kernel modules.
-  linuxPackages = linuxPackages_4_19;
+  linuxPackages = linuxPackages_5_4;
   linux = linuxPackages.kernel;
 
   # Update this when adding the newest kernel major version!