Bump flannel to v0.26.7

[brandond]

Bump flannel for this fix, and others:

• Use peer's wireguard port, not our own flannel-io/flannel#2200

flannel-io/flannel@v0.25.7...v0.26.7

[VestigeJ]

Existing Release Version

$ k3s -v

k3s version v1.32.3+k3s1 (079ffa8d)
go version go1.23.6

$ sudo /var/lib/rancher/k3s/data/cni/flannel --version

CNI Plugin flannel version v1.6.0-flannel1+v0.25.7 (linux/amd64) commit HEAD built on 2025-03-25T22:08:26Z
CNI protocol versions supported: 0.1.0, 0.2.0, 0.3.0, 0.3.1, 0.4.0, 1.0.0, 1.1.0

[VestigeJ]

Closing validated subsequent issues