linuxPackages: 6.1 -> 6.6

The latest longterm kernel release series is 6.6, and we should roll it
out as the default now, to receive as much testing as possible before
the NixOS 24.05 release.

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

@@ -16,6 +16,8 @@ In addition to numerous new and upgraded packages, this release has the followin
 
 - `linuxPackages_testing_bcachefs` is now fully deprecated by `linuxPackages_latest`, and is therefore no longer available.
 
+- The default kernel package has been updated from 6.1 to 6.6. All supported kernels remain available.
+
 - NixOS now installs a stub ELF loader that prints an informative error message when users attempt to run binaries not made for NixOS.
    - This can be disabled through the `environment.stub-ld.enable` option.
    - If you use `programs.nix-ld.enable`, no changes are needed. The stub will be disabled automatically.

pkgs/top-level/linux-kernels.nix

@@ -667,7 +667,7 @@ in {
   });
 
   packageAliases = {
-    linux_default = packages.linux_6_1;
+    linux_default = packages.linux_6_6;
     # Update this when adding the newest kernel major version!
     linux_latest = packages.linux_6_7;
     linux_mptcp = throw "'linux_mptcp' has been moved to https://github.com/teto/mptcp-flake";