change default ipv6 prefix to 3fff:172:20:20::/64

[hyposcaler-bot]

This change resolves #2229 by updating the /64 used for management addresses to 3fff:172:20:20::/64

For more details on the documentation prefix see IANA IPv6 Special-Purpose Address Registry and RFC9637

Change has only been tested lightly, only really confirmed that new prefix works by spinning up a simple single node lab and confirming that mgmt address was provisioned from new /64 and it was reachable.

dfanshaw@observer:~/src/network/labs/clab/simple-eos$ cat simple-eos.yml
name: eos-simple
topology:
  nodes:
    clab-eos-switch:
            kind: ceos
            image: ceos64-lab-4.33.0f
dfanshaw@observer:~/src/network/labs/clab/simple-eos$ sudo ~/src/network/containerlab/bin/containerlab deploy -t simple-eos.yml
INFO[0000] Containerlab v0.0.0 started
INFO[0000] Parsing & checking topology file: simple-eos.yml
INFO[0000] Creating docker network: Name="clab", IPv4Subnet="172.20.20.0/24", IPv6Subnet="3fff:172:20:20::/64", MTU=1500
WARN[0000] Unable to load kernel module "ip6_tables" automatically "load ip6_tables failed: exec format error"
INFO[0000] Creating lab directory: /home/dfanshaw/src/network/labs/clab/simple-eos/clab-eos-simple
INFO[0000] config file '/home/dfanshaw/src/network/labs/clab/simple-eos/clab-eos-simple/clab-eos-switch/flash/startup-config' for node 'clab-eos-switch' already exists and will not be generated/reset
INFO[0000] Creating container: "clab-eos-switch"
INFO[0000] Running postdeploy actions for Arista cEOS 'clab-eos-switch' node
INFO[0013] Adding containerlab host entries to /etc/hosts file
INFO[0013] Adding ssh config for containerlab nodes
INFO[0013] 🎉 New containerlab version 0.57.5 is available! Release notes: https://containerlab.dev/rn/0.57/#0575
Run 'containerlab version upgrade' to upgrade or go check other installation options at https://containerlab.dev/install/
+---+---------------------------------+--------------+--------------------+------+---------+----------------+----------------------+
| # |              Name               | Container ID |       Image        | Kind |  State  |  IPv4 Address  |     IPv6 Address     |
+---+---------------------------------+--------------+--------------------+------+---------+----------------+----------------------+
| 1 | clab-eos-simple-clab-eos-switch | 2361f61083f5 | ceos64-lab-4.33.0f | ceos | running | 172.20.20.2/24 | 3fff:172:20:20::2/64 |
+---+---------------------------------+--------------+--------------------+------+---------+----------------+----------------------+
dfanshaw@observer:~/src/network/labs/clab/simple-eos$ ssh admin@3fff:172:20:20::2
The authenticity of host '3fff:172:20:20::2 (3fff:172:20:20::2)' can't be established.
ED25519 key fingerprint is SHA256:w3XHLu3Qn8xsB8Txl7x9yWzpQZaD6cl/GoQxE4ZPNKE.
This host key is known by the following other names/addresses:
    ~/.ssh/known_hosts:1: [hashed name]
Are you sure you want to continue connecting (yes/no/[fingerprint])? yes
Warning: Permanently added '3fff:172:20:20::2' (ED25519) to the list of known hosts.
(admin@3fff:172:20:20::2) Password:
clab-eos-switch>show ipv6 int br
   Interface       Status        MTU       IPv6 Address                     Addr State    Addr Source
--------------- ------------ ---------- -------------------------------- ---------------- -----------
   Ma0             up           1500       fe80::42:acff:fe14:1402/64       up            link local
                                           3fff:172:20:20::2/64             up            config

clab-eos-switch>

This PR makes no changes to documentation.

[hellt]

thnx @hyposcaler-bot
I've changed the doc entries as well, looks good, shipping

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 51.98%. Comparing base (7c511e0) to head (eddaea9).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2232   +/-   ##
=======================================
  Coverage   51.98%   51.98%           
=======================================
  Files         172      172           
  Lines       12658    12658           
=======================================
  Hits         6580     6580           
  Misses       5171     5171           
  Partials      907      907           

Files with missing lines	Coverage Δ
clab/config.go	68.51% <ø> (ø)