private DeploymentSpec.UpgradePolicy readUpgradePolicy(String policy) {
        return switch (policy) {
            case "canary" -> UpgradePolicy.canary;
            case "default" -> UpgradePolicy.defaultPolicy;
            case "conservative" -> UpgradePolicy.conservative;
            default -> throw new IllegalArgumentException("Illegal upgrade policy '" + policy + "': " +
                                                          "Must be one of 'canary', 'default', 'conservative'");
        };
    }
Returns true if the given value is "true", or if it is missing
config-model-api/src/main/java/com/yahoo/config/application/api/xml/DeploymentSpecXmlReader.java