gokrazy, tstest, cmd/vnet: switch amd64 kernel to gokrazy/kernel.amd64

The tailscale/gokrazy-kernel module was a fork of rtr7/kernel that
stalled at Linux 6.8.9 (July 2024). All of the kernel config options we
had added in that fork (ENA, Xen for EC2, virtio-mmio for qemu microvm,
virtio RNG, IPv6 policy routing, netlink diag, etc) are now present in
the gokrazy project's own gokrazy/kernel.amd64 module, which tracks
current kernel.org releases (Linux 7.1.3 as of this change) and is the
gokrazy project's supported kernel for x86_64 PCs and VMs.

Switch the tsapp and natlabapp images, the natlab VM tests, and the
CI workflow to gokrazy/kernel.amd64, drop the tailscale/gokrazy-kernel
dependency, and update gokrazy/kernel.arm64 to latest while here.

Verified with TestEasyEasy, TestJustIPv6, and TestTailscaleSSH in
tstest/natlab/vmtest with --run-vm-tests.

Updates #1866

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
Change-Id: I90c3765a4e18f5609b4d77b51ac38d17c8e3688a

.github/workflows/natlab-test.yml

@@ -165,13 +165,13 @@ jobs:
           key: natlab-gokrazy-${{ github.sha }}
 
       # The gokrazy-based tests boot the kernel directly from
-      # vmlinuz that ships in the tailscale/gokrazy-kernel module.
+      # vmlinuz that ships in the gokrazy/kernel.amd64 module.
       # Tests look it up under GOMODCACHE via findKernelPath, so the
       # module has to be present even though no Go source imports it
       # in the test package itself.
-      - name: Download gokrazy-kernel module
+      - name: Download kernel.amd64 module
         run: |
-          ./tool/go mod download github.com/tailscale/gokrazy-kernel
+          ./tool/go mod download github.com/gokrazy/kernel.amd64
 
       - name: Run ${{ matrix.test }}
         # Per-test timeout is well above the few-minute typical runtime