Need to upgrade wg-easy bootstrapper script

WireGuard Easy repo: https://github.com/wg-easy/wg-easy
* [v15.0.0](https://github.com/wg-easy/wg-easy/releases/tag/v15.0.0) was release in May 28, 2025.

The previous wg-easy bootstrapper script used v13.
https://github.com/cloud-barista/mc-terrarium/tree/main/examples/aws/client-to-site-vpn/wireguard-easy