linuxPackages: 5.4 -> 5.10

The 5.10 series is the next longterm version of the linux kernel and
I've been using it on multiple x86_64 machines ever since it came out.

I think it is time to switch over the default now, so we get some
additional testing in time for NixOS 21.05.

nixos/doc/manual/release-notes/rl-2105.xml

@@ -23,6 +23,9 @@
      Support is planned until the end of December 2021, handing over to 21.11.
     </para>
    </listitem>
+   <listitem>
+    <para>The default Linux kernel was updated to the 5.10 LTS series, coming from the 5.4 LTS series.</para>
+   </listitem>
    <listitem>
     <para>GNOME desktop environment was upgraded to 3.38, see its <link xlink:href="https://help.gnome.org/misc/release-notes/3.38/">release notes</link>.</para>
    </listitem>

pkgs/top-level/all-packages.nix

@@ -19891,7 +19891,7 @@ in
   });
 
   # The current default kernel / kernel modules.
-  linuxPackages = linuxPackages_5_4;
+  linuxPackages = linuxPackages_5_10;
   linux = linuxPackages.kernel;
 
   # Update this when adding the newest kernel major version!