private DeploymentSpec.RevisionChange readRevisionChange(String revision) {
        return switch (revision) {
            case "when-clear" -> RevisionChange.whenClear;
            case "when-failing" -> RevisionChange.whenFailing;
            case "always" -> RevisionChange.always;
            default -> throw new IllegalArgumentException("Illegal upgrade revision change policy '" + revision + "': " +
                                                          "Must be one of 'always', 'when-failing', 'when-clear'");
        };
    }
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