Convert Depreacted Flags for v1.27

[dereknola]

Component of #5940, with v1.27, we need to completely remove the following flags from k3s and docs.

• wireguard backend
• k3s etcd-snapshot will no longer save a snapshot, it is now prints the help message.

Additionally, we must mark the following flags with a fatal error:

• ipsec backend
• Additional options of flannel-backend

[ShylajaDevadiga]

Validated using v1.27.1-rc1+k3s1

$ k3s etcd-snapshot
NAME:
   k3s etcd-snapshot - Trigger an immediate etcd snapshot

USAGE:
   k3s etcd-snapshot command [command options] [arguments...]

Shows FATAL error passed --flannel-backend=ipsec as expected

ubuntu@ip-172-31-3-176:~$ sudo k3s server --flannel-backend=ipsec 
...
FATA[0000] The ipsec backend is deprecated and was removed in k3s v1.27; please switch to wireguard-native. Check our docs for information on how to migrate. 

We need to updated the help text with removal of the flags

$ k3s server --help |grep flannel
   --flannel-backend value                    (networking) backend<=option1=val1,option2=val2> where backend is one of 'none', 'vxlan', 'ipsec' (deprecated), 'host-gw', 'wireguard-native', 'wireguard' (deprecated) (default: "vxlan")
   --flannel-ipv6-masq                        (networking) Enable IPv6 masquerading for pod
   --flannel-external-ip                      (networking) Use node external IP addresses for Flannel traffic
   --flannel-iface value                      (agent/networking) Override default flannel interface
   --flannel-conf value                       (agent/networking) Override default flannel config file
   --flannel-cni-conf value                   (agent/networking) Override default flannel cni config file

[ShylajaDevadiga]

Validated help text is updated using the commit bc5b42c from master branch

$ sudo k3s server --help|grep flannel
   --flannel-backend value                    (networking) Backend (valid values: 'none', 'vxlan', 'ipsec' (deprecated), 'host-gw', 'wireguard-native' (default: "vxlan")
   --flannel-ipv6-masq                        (networking) Enable IPv6 masquerading for pod
   --flannel-external-ip                      (networking) Use node external IP addresses for Flannel traffic
   --flannel-iface value                      (agent/networking) Override default flannel interface
   --flannel-conf value                       (agent/networking) Override default flannel config file
   --flannel-cni-conf value                   (agent/networking) Override default flannel cni config file