storage: release v1.6.0
storage: release v1.6.0

This commit will be tagged storage/v1.6.0

Change-Id: I4eb22c0cbd9b8ba9672098ca26b121e5cec2355c
Reviewed-on: https://code-review.googlesource.com/c/gocloud/+/52413
Reviewed-by: Chris Cotter <b879ae7d4b52f11ce9f60d9854075eb8eebaf7a9@google.com>

googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go