public boolean canUpgradeAt(Instant instant) {
        return changeBlockers.stream().filter(DeploymentSpec.ChangeBlocker::blocksVersions)
                                      .noneMatch(block -> block.window().includes(instant));
    }
Returns whether the instances in this step can upgrade at the given instant
config-model-api/src/main/java/com/yahoo/config/application/api/DeploymentInstanceSpec.java