External Network: Wireguard MTU

[cheina97]

Description

This PR set up the wireguard default MTU.

https://docs.tigera.io/calico/latest/networking/configuring/mtu#:~:text=Recommended%20MTU%20for%20overlay%20networking&text=WireGuard%20sets%20the%20Don't,network%20to%20avoid%20dropped%20packets.

[adamjensenbot]

Hi @cheina97. Thanks for your PR!

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