Update the calico_veth_mtu setting to affect IP-in-IP users (#6419)

* Update calico_veth_mtu to FELIX_IPINIP variable

calico_veth_mtu is specified in the configuration, but since it only works for wireguard, modify it to work for IP-in-IP users.

* Update template with more cleaner expression

roles/network_plugin/calico/templates/calico-node.yml.j2

@@ -232,7 +232,7 @@ spec:
             # Set MTU for tunnel device used if ipip is enabled
 {% if calico_mtu is defined %}
             - name: FELIX_IPINIPMTU
-              value: "{{ calico_mtu }}"
+              value: "{{ calico_veth_mtu | default(calico_mtu) }}"
 {% endif %}
             - name: FELIX_CHAININSERTMODE
               value: "{{ calico_felix_chaininsertmode }}"