[Release-1.29] - Bump etcd to v3.5.21

[brandond]

Backport fix for Bump etcd to v3.5.21

• Bump etcd to v3.5.21 #12116

[VestigeJ]

🟢 k3s -v
k3s version v1.29.15+k3s-7867eb64 (7867eb6)
go version go1.23.6

$ sudo journalctl -u k3s | grep -i etcd-version

"etcd-version":"3.5.21"