Update rook/cassandra Docker tag to v1.7.3

This MR contains the following updates:

Package	Type	Update	Change
rook/cassandra	ironbank-docker	patch	v1.7.2 -> v1.7.3
rook/cassandra		patch	v1.7.2 -> v1.7.3
rook/cassandra	stage	patch	v1.7.2 -> v1.7.3

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