Upgrade Cilium to v1.15.6

[rifelpet]

We'll see if this fixes #16597

https://github.com/cilium/cilium/releases/tag/v1.15.6
https://github.com/cilium/cilium/releases/tag/v1.15.5

[hakman]

/retest
/lgtm
/approve

[k8s-ci-robot]

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[k8s-ci-robot]

@rifelpet: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-kops-e2e-cni-cilium-ipv6	ecac334	link	false	/test pull-kops-e2e-cni-cilium-ipv6

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