Update quay.io/cilium/cilium Docker tag to v1.15.5

This MR contains the following updates:

Package	Type	Update	Change
quay.io/cilium/cilium		patch	v1.15.4 -> v1.15.5
quay.io/cilium/cilium	ironbank-docker	patch	v1.15.4 -> v1.15.5
quay.io/cilium/cilium	stage	patch	v1.15.4 -> v1.15.5

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