Convert Deprecated Flags for v1.28

[dereknola]

Component of #5940, with v1.28, we need to completely remove the following flags from k3s and docs.

• ipsec backend
• Additional options of flannel-backend

Also #7408

[ShylajaDevadiga]

Validated using k3s version v1.28.1-rc2+k3s1

• ipsec is removed from the help text
• installation with flannel-backend=ipsec fails

$ curl -fL https://get.k3s.io| INSTALL_K3S_VERSION=v1.28.1-rc2+k3s1 sh -s - server --token secret --flannel-backend=ipsec

On k3s version v1.27.4+k3s1

$ k3s server --help |grep backend 
   --flannel-backend value                    (networking) Backend (valid values: 'none', 'vxlan', 'ipsec' (deprecated), 'host-gw', 'wireguard-native' (default: "vxlan")

From the logs:

Sep 06 19:49:11 ip-172-31-0-244 k3s[8595]: time="2023-09-06T19:49:11Z" level=fatal msg="The ipsec backend is deprecated and was removed in k3s v1.27; please switch to wireguard-native. Check>
Sep 06 19:49:11 ip-172-31-0-244 systemd[1]: k3s.service: Main process exited, code=exited, status=1/FAILURE

On k3s version v1.28.1-rc2+k3s1

$ k3s server --help |grep backend
   --flannel-backend value                    (networking) Backend (valid values: 'none', 'vxlan', 'host-gw', 'wireguard-native' (default: "vxlan")

From the logs:

Sep 06 19:44:08 ip-172-31-0-244 k3s[7337]: time="2023-09-06T19:44:08Z" level=fatal msg="Cannot configure unknown flannel backend 'ipsec'"
Sep 06 19:44:08 ip-172-31-0-244 systemd[1]: k3s.service: Main process exited, code=exited, status=1/FAILURE