neonvm-kernel: Switch to version 6.12.8 (#1193)

This commit updates the kernel and switches from 6.6.x to 6.12.x LTS
branch.

As of 2025-01-02, www.kernel.org says this is the most recent version of
the 6.12.x LTS branch.

The upgrade is done according to the instruction in
neonvm-kernel/README.md.

ref neondatabase/cloud#20195

neonvm-kernel/README.md

@@ -29,17 +29,17 @@ with the following sequence of actions:
 2. Then, inside the container, run:
    ```sh
    cd linux-$NEW_VERSION
-   cp /host/linux-config-amd64-6.6.64 .config # Copy current config in
+   cp /host/linux-config-amd64-6.12.8 .config # Copy current config in
    make menuconfig ARCH=x86_64
    # do nothing; just save and exit, overwriting .config
    cp .config /host/linux-config-amd64-$NEW_VERSION # NOTE: Different from existing!
    ```
 3. Back on the host, finish with:
    ```sh
    # compare the two versions
-   diff linux-config-amd64-6.6.64 linux-config-amd64-$NEW_VERSION
+   diff linux-config-amd64-6.12.8 linux-config-amd64-$NEW_VERSION
    # If all looks good, delete the old version. This is required so auto-selection works.
-   rm linux-config-amd64-6.6.64
+   rm linux-config-amd64-6.12.8
    ```
 
 ### On arm64 (aarch64 ARM)
@@ -53,17 +53,17 @@ with the following sequence of actions:
 2. Then, inside the container, run:
    ```sh
    cd linux-$NEW_VERSION
-   cp /host/linux-config-aarch64-6.6.64 .config # Copy current config in
+   cp /host/linux-config-aarch64-6.12.8 .config # Copy current config in
    make menuconfig ARCH=arm64
    # do nothing; just save and exit, overwriting .config
    cp .config /host/linux-config-aarch64-$NEW_VERSION # NOTE: Different from existing!
    ```
 3. Back on the host, finish with:
    ```sh
    # compare the two versions
-   diff linux-config-aarch64-6.6.64 linux-config-aarch64-$NEW_VERSION
+   diff linux-config-aarch64-6.12.8 linux-config-aarch64-$NEW_VERSION
    # If all looks good, delete the old version. This is required so auto-selection works.
-   rm linux-config-aarch64-6.6.64
+   rm linux-config-aarch64-6.12.8
    ```
 
 Afterwards, it's probably also good to do a search-and-replace repo-wide to update all places that
@@ -74,7 +74,7 @@ mention the old kernel version.
 To adjust the kernel config, try the following from this directory:
 
 ```sh
-docker build --build-arg KERNEL_VERSION=6.6.64 --platform linux/x86_64 --target build-deps -t kernel-build-deps -f Dockerfile.kernel-builder .
+docker build --build-arg KERNEL_VERSION=6.12.8 --platform linux/x86_64 --target build-deps -t kernel-build-deps -f Dockerfile.kernel-builder .
 docker run --rm -v $PWD:/host --name kernel-build -it kernel-build-deps bash
 # inside that bash shell, do the menuconfig, then copy-out the config to /host
 ```