[coreos-status] Fedora CoreOS moving to iptables-nft

[Jonathan Lebon]

In coming months, all new and upgrading Fedora CoreOS nodes will
migrate to the nft backend of iptables. This will be done by updating
the relevant symbolic links in `/etc/alternatives`. The legacy backend
is considered deprecated.

This change will be rolled out together with the rebase to Fedora 36:
    - the `next` rebase is targeted for 2022-03-15
    - the `testing` rebase is targeted for 2022-04-19
    - the `stable` stream will follow `testing` as usual

If you need to stay on the legacy backend, create an empty file at
`/etc/coreos/iptables-legacy.stamp`. For existing nodes, you can
manually create the file now:

```
sudo mkdir -m 755 /etc/coreos/
sudo touch /etc/coreos/iptables-legacy.stamp
```

For new nodes that get deployed between now and when the migration
happens, you can create the `/etc/coreos/iptables-legacy.stamp` file
using Ignition to ensure they don't get migrated. After the migration,
you can bring up new nodes on the legacy backend by manually setting
the symbolic links via Ignition. Below is a Butane config that does
both of these:

```
variant: fcos
version: 1.4.0
storage:
  files:
    - path: /etc/coreos/iptables-legacy.stamp
      mode: 0644
  links:
    - path: /etc/alternatives/iptables
      target: /usr/sbin/iptables-legacy
      overwrite: true
      hard: false
    - path: /etc/alternatives/iptables-restore
      target: /usr/sbin/iptables-legacy-restore
      overwrite: true
      hard: false
    - path: /etc/alternatives/iptables-save
      target: /usr/sbin/iptables-legacy-save
      overwrite: true
      hard: false
    - path: /etc/alternatives/ip6tables
      target: /usr/sbin/ip6tables-legacy
      overwrite: true
      hard: false
    - path: /etc/alternatives/ip6tables-restore
      target: /usr/sbin/ip6tables-legacy-restore
      overwrite: true
      hard: false
    - path: /etc/alternatives/ip6tables-save
      target: /usr/sbin/ip6tables-legacy-save
      overwrite: true
      hard: false
```

This will ensure that all new nodes will use the legacy backend
whether before or after the migration. After all streams are based on
Fedora 36, we recommend removing the stamp file from your Butane
config.

Thanks,

Jonathan Lebon, for the Fedora CoreOS team