Update cockroachdb/cockroach Docker tag to v21.2.6

This MR contains the following updates:

Package	Type	Update	Change
cockroachdb/cockroach	ironbank-docker	patch	v21.2.5 -> v21.2.6
cockroachdb/cockroach		patch	v21.2.5 -> v21.2.6
cockroachdb/cockroach	stage	patch	v21.2.5 -> v21.2.6

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