Update docker.io/prom/memcached-exporter Docker tag to v0.15.4

This MR contains the following updates:

Package	Type	Update	Change
docker.io/prom/memcached-exporter	ironbank-docker	patch	v0.15.3 -> v0.15.4
docker.io/prom/memcached-exporter	final	patch	v0.15.3 -> v0.15.4

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