docs/reference: remove mention of DOCKER_NOWARN_KERNEL_VERSION

Support for this environment variable was removed in docker 23.0 in
moby/moby@1240f8b

From that patch:

> All regular, non-EOL Linux distros now come with more recent kernels
> out of the box. There may still be users trying to run on kernel 3.10
> or older (some embedded systems, e.g.), but those should be a rare
> exception, which we don't have to take into account.
>
> This patch removes the kernel version check on Linux, and the corresponding
> DOCKER_NOWARN_KERNEL_VERSION environment that was there to skip this
> check.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

Author: thaJeztah

docs/reference/commandline/dockerd.md

@@ -130,8 +130,6 @@ For easy reference, the following list of environment variables are supported
 by the `dockerd` command line:
 
 * `DOCKER_DRIVER` The graph driver to use.
-* `DOCKER_NOWARN_KERNEL_VERSION` Prevent warnings that your Linux kernel is
-  unsuitable for Docker.
 * `DOCKER_RAMDISK` If set this will disable 'pivot_root'.
 * `DOCKER_TMPDIR` Location for temporary Docker files.
 * `MOBY_DISABLE_PIGZ` Do not use [`unpigz`](https://linux.die.net/man/1/pigz) to