fix: surround IPv6 addresses with brackets

[lion7]

The host IP address is passed to netip.ParseAddrPort, which returns an error if IPv6 addresses are not surrounded in brackets. This happens when the Kubernetes node has an IPv6 address. Errors I saw in my logs:

❯ kubectl logs -n sidero-system sidero-controller-manager-64788bbf65-w42f7 siderolink 
I0428 12:29:44.434812       1 request.go:601] Waited for 1.04738863s due to client-side throttling, not priority and fairness, request: GET:https://[2a02:****:****:1000::11:1]:443/apis/cilium.io/v2?timeout=32s
error: invalid Wireguard endpoint: invalid ip:port "2a02:****:****:0:d250:99ff:fefa:a836:51821", IPv6 addresses must be surrounded by square brackets

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[smira]

@lion7 thanks for reporting this, I have a simpler fix in #1122