Return to the vanilla kernel

Our choice of the Zen kernel was a bit random and speculative. So far we
have a report that it hurts something, and no reports that it helps
anything.

Over time the differences between the Zen kernel and the vanilla one
have dwindled, and I can barely find an up-to-date list of the
differences without looking through source code.

I would consider the upstream vanilla sources to be safer and more
trustworthy, and I expect will be much easier to get through shim
review. Let's return to them.

Resolves #601

mkosi.conf.d/00-packages-core.conf

@@ -17,8 +17,8 @@ Packages=
     kmod
     kmscon # modern userspace getty and virtual terminal replacement
     linux-firmware
-    linux-zen-headers # For DKMS
-    linux-zen # Improved Kernel
+    linux-headers # For DKMS
+    linux # It's the kernel!
     plymouth
     realtime-privileges # `realtime` group and realtime privileges for processes using it
     shim # Sits between firmware and systemd-boot, providing a trusted boot path and support for Secure Boot
@@ -35,7 +35,7 @@ Packages=
     libva-mesa-driver # Hardware accelerated video on AMD GPUs
     libva-nvidia-driver # Hardware accelerated video on NVIDIA GPUs
     mesa
-    nvidia-open-dkms # For all Linux Kernels; appears to contain vulkan support already
+    nvidia-open # Open kernel modules for NVIDIA GPUs w/ vulkan support
     vpl-gpu-rt # Hardware accelerated video on newer Intel GPUs
     vulkan-icd-loader
     vulkan-intel